Calappa flammeaPLATE XCalappa flammea,BRAZIL. (REDUCED)View larger image
PLATE X
Calappa flammea,BRAZIL. (REDUCED)
View larger image
TheOxystomata(Plate X.), which form the second tribe of the Brachyura, are distinguished by the form of the mouth-frame, which is narrowed in front so as to be triangular instead of square in outline. The passages through which the water passes out from the gills, which in other Crabs open at the front corners of the mouth-frame, are carried forwards to the front of the head. The Oxystomata are most abundant in tropical seas, but are represented on the British coasts by species ofEbalia, small and compact Crabs which are not unlike pebbles of the gravel among which they live.
The remaining Crabs form the tribeBrachygnatha, in which the mouth-frame and the maxillipeds that close it are more or less quadrilateral in shape.The tribe is divided into two subtribes, which may be recognized by the general shape of the carapace. In theBrachyrhynchathis is generally rounded or square-cut in front, without a projecting rostrum. In this subtribe are included the great majority of Crabs. The Edible Crab and the Shore Crab (Plate IX.) are familiar examples. In theOxyrhyncha, on the other hand, the carapace is generally narrowed in front, with a projecting rostrum, either simple or forked, and is often armed with spines. In this subtribe are included the long-legged Spider Crabs, several species of which are common on our coasts. The Giant Spider Crab of Japan (Plate XI.) is the largest of living Crustacea.
Calappa flammeaPLATE XITHE GIANT JAPANESE CRAB,Macrocheira kæmpferi,MALE.THE SCALE OF THE FIGURE IS GIVEN BY A TWO-FOOT RULE PLACED BELOW THE SPECIMEN(From Brit. Mus. Guide)View larger image
PLATE XI
THE GIANT JAPANESE CRAB,Macrocheira kæmpferi,MALE.THE SCALE OF THE FIGURE IS GIVEN BY A TWO-FOOT RULE PLACED BELOW THE SPECIMEN
(From Brit. Mus. Guide)
View larger image
The last division of the Eumalacostraca, theHoplocarida(Plate XII.), is one of very small extent, comprising only a single order (Stomatopoda) of very remarkable Crustacea which are common in tropical seas, and of which at least one species,Squilla desmarestii, is occasionally captured on the south coast of England. The Stomatopoda are prawn-like Crustaceans, usually with a flattened body, and are easily recognized by the form of the large claws (the second pair of thoracic limbs), in which the last segment shuts down, like the blade of a pocket-knife, on the preceding segment, and forms a very efficient weapon, so that the larger species are not to be handled without caution. The resemblance of these claws to those of the mantis-insectof Southern Europe led to a common Mediterranean species receiving long ago the nameSquilla mantis(Plate XII.).
Squilla mantisPLATE XIISquilla mantis,FROM THE MEDITERRANEAN. ABOUT ONE-HALF NATURAL SIZE(From Brit. Mus. Guide)View larger image
PLATE XII
Squilla mantis,FROM THE MEDITERRANEAN. ABOUT ONE-HALF NATURAL SIZE
(From Brit. Mus. Guide)
View larger image
The Stomatopoda have a small carapace, which does not cover the last four thoracic somites, and has in front a small flattened rostrum, attached by a movable hinge, like that of the Leptostraca. The eyes are stalked, and, like the antennules, are attached to a separate movable segment of the front part of the head—a peculiarity not found in any other Crustacea. There are small plate-like gills attached to the bases of some of the thoracic limbs, but the chief organs of respiration are large feathery gills attached to the pleopods or swimmerets.
The Stomatopoda are all found in the sea, generally in shallow water, burrowing in sand or hiding in crevices of rocks or corals. Some species are more than a foot in length.
The great majority of Crustacea are hatched from the egg in a form very different from that which they finally assume, and reach the adult state only after passing through a series of transformations quite as remarkable as those which a caterpillar undergoes in becoming a butterfly, or a tadpole in becoming a frog. Many of these young stages were known for a long time before their larval nature was suspected, and it is one of the curiosities of the history of zoology that, even after the actual changes from one form to another had been observed and described in several Crustacea, many eminent naturalists refused to believe in the possibility of their occurrence. This scepticism was largely due to the fact that the common fresh-water Crayfish, when hatched from the egg, has practically the same structure as the adult, and it was assumed that other Crustacea were developed in a similar fashion. Although certain cases of metamorphosis had been actually seen and described by naturalists in the eighteenth century, these observations were forgottenor misunderstood till they were confirmed by Mr. J. Vaughan Thompson, a naval surgeon stationed at Cork, the first part of whose "Zoological Researches" was published in 1828. Thompson's statements were much disputed at the time, but they have been confirmed by subsequent research, and it is now known that the majority of Crustacea undergo a more or less extensive metamorphosis after leaving the egg, although, as will be seen later, there are many important exceptions to this rule.
If a fine muslin net be towed at the surface of the sea on a calm day, and the contents turned out into a jar of sea-water, it will usually be found to have captured, among other things, clouds of animated specks, which dance in the water or dart hither and thither with great rapidity. Many of these specks, when examined with the microscope, will be found to be Crustacea. Besides adult animals belonging to various groups, such as the Copepoda, which pass the whole of their life swimming near the surface of the sea, there will be numerous larval stages of species which in their adult form live on the sea-bottom. The identification of the species to which the various larvæ belong is a matter of considerable difficulty, and, although the general course of development is now well known for all the chief groups of Crustacea, there are very many even of the common British species in which the larval transformations have not yet been worked out in detail.
Larval Stages of the Common Shore CrabFig. 25—Larval Stages of the Common Shore Crab(Carcinus mænas—seePlate IX.). (Partly after Williamson.)A, Young zoëa, shortly after hatching; B, megalopa stage; C, young Crab. A × 20, B and C × 10View larger image
Fig. 25—Larval Stages of the Common Shore Crab(Carcinus mænas—seePlate IX.). (Partly after Williamson.)
A, Young zoëa, shortly after hatching; B, megalopa stage; C, young Crab. A × 20, B and C × 10
View larger image
As an example of the larvalhistory of the higher Crustacea, we may take the case of the Common Shore Crab,Carcinus mænas(Fig. 25). The young stages are common in tow-net gatherings round the British coasts in the summer-time. The youngest larvæ (Fig. 25, A) are translucent little creatures about one-twentieth of an inch long. They have the head and front part of the body covered by a helmet-shaped carapace, with a long spine standing out from the middle of the back, and anotherprojecting, like a beak, in front.
The narrow abdomen or tail is very flexible, and can be doubled up under the body or stretched out behind; it ends in a forked telson. There are two pairs of swimming limbs, each with endopodite and exopodite, and the short antennules and antennæ are seen on either side of the rostrum. There are a pair of very large compound eyes, which are not set on movable stalks, but are under the front part of the carapace. The two-branched swimming feet are really the first and second maxillipeds (the mandibles, maxillulæ, and maxillæ, can be found in front of them), but none of the other thoracic limbs are yet developed, and, although the somites of the abdomen are distinct, there are no swimmerets. This type of larva is known as azoëa, a name which was given to it when it was supposed to be an independent species of Crustacean. As a matter of fact, the zoëa just described is not quite the earliest stage of the Shore Crab, for when hatched from the egg it is without the spines on the carapace, and is slightly different in other respects. A few hours after hatching, however, it casts its skin for the first time, and becomes a fully-formed zoëa. It swims rapidly about at the surface of the sea, feeding on the minute floating animals and plants which are found there, and growing in size with repeated castings of its skin. In the later stages of the zoëa the rudimentsof the hinder thoracic limbs and of the swimmerets appear as little buds. In the next stage (Fig. 25, B) all the appendages are present, the dorsal spine of the carapace has disappeared, the eyes are stalked and movable, and the animal has all the appearance of a little Crab, except that the abdomen is stretched out instead of being tucked up under the body, and the swimmerets are used as paddles in swimming. In this stage the larva, which is known as amegalopa, swims at the surface of the sea, but later it sinks to the bottom, and, moulting again, appears as a little Crab (Fig. 25, C), with tucked-up abdomen and swimmerets no longer adapted for locomotion.
Last Larval Stage of the Common Porcelain CrabFig. 26—Last Larval Stage of the Common Porcelain Crab(Porcellana longicornis—seeFig. 41, p. 113). × 9. (After Sars.)View larger image
Fig. 26—Last Larval Stage of the Common Porcelain Crab(Porcellana longicornis—seeFig. 41, p. 113). × 9. (After Sars.)
View larger image
First Larval Stage of Munida rugosaFig. 27—First Larval Stage ofMunida rugosa(seePlate VI.). × 10. (After Sars.)View larger image
Fig. 27—First Larval Stage ofMunida rugosa(seePlate VI.). × 10. (After Sars.)
View larger image
Most of the true Crabs (Brachyura) have a larval history similar to that just described, and pass through zoëa and megalopa stages which differ only in details from those ofCarcinus. The Anomura are also hatched as zoëæ, and one of the most remarkable forms common in tow-nettings in British waters is the zoëa of the little Porcelain Crabs (Porcellana—Fig. 26). In this larva the carapace has two long spinesbehind, and a rostral spine which is several times as long as the body of the animal. A great development of spines also characterizes the larva ofMunida(Fig. 27).
The larval form of the Common Lobster has already been described, and it will be noticed that the differences from the adult are much less than in the case of the Crab. From the fact that this larva has swimming exopodites on its legs, like the adult Mysidacea and Euphausiacea (formerly grouped together as "Schizopoda"), it is said to be in the "schizopod stage." The larva of the Norway Lobster (Nephrops norvegicus) is essentially of the same type, but the great development of the spines on the abdomen and of the forked telson gives it a striking appearance.
A very remarkable type of larva is found among the Spiny Lobsters and their allies (Scyllaridea).This larva, known by the name ofphyllosoma(Fig. 28), is very broad, thin, and leaf-like, and quite transparent, so that some of the larger kinds were formerly known as "Glass Crabs." The thin oval carapace does not cover the whole of the thoracic region, which is disc-shaped, with four pairs of long slender legs, each with an exopodite. The abdomen is relatively small. The intermediate stages between the phyllosoma and the adult are still very imperfectly known. In tropical seas phyllosoma larvæ of large size are found, sometimes reaching two or three inches in length. The larva of the Common Spiny Lobster (Palinurus vulgaris), however, does not exceed half an inch in length.
The Phyllosoma Larva of the Common Spiny LobsterFig. 28—The Phyllosoma Larva of the Common Spiny Lobster(Palinurus vulgaris—seePlate V.).Much enlarged.(After J. T. Cunningham.)View larger image
Fig. 28—The Phyllosoma Larva of the Common Spiny Lobster(Palinurus vulgaris—seePlate V.).Much enlarged.(After J. T. Cunningham.)
View larger image
The Shrimps and Prawns of the tribe Caridea are mostly hatched as zoëæ,and pass through a "schizopod" stage comparable to that of the Lobster, in which they swim by means of exopodites on the legs. Some of the Prawns belonging to the tribe Penæidea, however, have a still more remarkable metamorphosis, which is very important on account of the resemblance of the earlier stages to those of the lower Crustacea. Fritz Müller discovered in 1863 thatPenæusis hatched from the egg as aNauplius(Fig. 29, A), a form of larva which was previously known among the Copepoda, Branchiopoda, and Cirripedes. The nauplius, unlike the larvæ which we have been considering, has an unsegmented body, and has only three pairs of limbs. The body is pear-shaped in outline, and near the front end is seen the median eye, sometimes called, from its presence in this type of larva, the "nauplius-eye"; the paired eyes are not yet developed. The three pairs of limbs are shown by their later development to be the antennules, antennæ, and mandibles; the first pair are unbranched, the second and third divided into exopodite and endopodite. It is interesting to notice that the antennæ and mandibles, which in the adult animal are so widely different that it is difficult to trace any resemblance between them, are in the nauplius almost identical in form. Further, the antennæ, instead of being placed in front of the mouth as in the adult, lie on either sideof it, and each has at its base a hooked spine which projects inwards and serves for seizing particles of food and passing them into the mouth; the antennæ of the nauplius, in fact, serve as jaws, while it is only later that the mandibles take on this function.
Larval Stages of the PrawnFig. 29—Larval Stages of the Prawn—Penæus(seePlate IV.). × 45. (After F. Müller.)A, Nauplius; B, young zoëa; C, older zoëa; D, early "schizopod" stageView larger image
Fig. 29—Larval Stages of the Prawn—Penæus(seePlate IV.). × 45. (After F. Müller.)
A, Nauplius; B, young zoëa; C, older zoëa; D, early "schizopod" stage
View larger image
In the further development of the larva, the body increases in length and becomes divided into somites which increase in number by new somites appearing behind those already marked off; the rudimentsof the limbs also appear in regular order from before backwards; the dorsal shield of the nauplius grows out into a carapace, beneath which the paired eyes begin to develop in front. Thus after passing throughmetanaupliusandprotozoëastages (Fig. 29, B) the larva becomes azoëa(Fig. 29, C), resembling that of the Crab already described in that the swimming organs are the maxillipeds, but differing in having the uropods well developed and forming a tail-fan at the end of the abdomen, the hinder thoracic somites marked off and their appendages present as rudiments, and the stalked eyes free from the carapace. This is followed by aschizopodstage (Fig. 29, D), in which the prawn-like shape is assumed and the thoracic legs have large exopodites used for swimming. Later these exopodites diminish in size, though they do not quite disappear in the adultPenæus, and the function of swimming organs is taken over by the abdominal swimmerets.
InPenæusthe larvæ are of comparatively simple form, but in the allied genusSergestesthe zoëa has a very remarkable appearance. The carapace is armed with long spines, each bearing two comb-like rows of secondary spines. The development of spines and other outgrowths of the surface of the body is a very common characteristic of organisms that, like these larvæ, float or swim in the open sea; its probable significance will be discussed in a later chapter.
The shrimp-like Euphausiacea have a larval development very like that ofPenæus. Most, if not all, of the species are hatched from the egg in the nauplius stage, and pass through stages very similar to those described above. The adult animals, however, may be said to remain in the "schizopod" stage, since the exopodites of the thoracic legs remain large and are used in swimming.
Newly-hatched Young of a CrayfishFig. 30—Newly-hatched Young of a Crayfish(Astacus fluviatilis).EnlargedView larger image
Fig. 30—Newly-hatched Young of a Crayfish(Astacus fluviatilis).Enlarged
View larger image
Even among the Decapoda, however, there are many species that are hatched from the egg in a form that does not differ essentially from the adult, and are therefore said to have a direct development. This is often the case with species which live in fresh water or in the depths of the sea. For example, the young of the fresh-water Crayfish (Fig. 30), when hatched, possess all the appendages of the adult except thefirst pair of swimmerets and the uropods, or outer plates of the tail-fan. The carapace is almost globular, owing to the presence inside the body of a large amount of food-yolk, which supplies the nourishment necessary for the young animal in the early stages of its development. The chelæ have hooked tips, by means of which the young animal clings securely to the swimmerets of the mother. After a time it moults, and the uropods are set free, the chelæ lose their hooked tips, the carapace assumes nearly its final shape (the food-yolk having been largely absorbed), and the young Crayfish leaves the protection of its parent, to shift for itself. The essential point of difference between the development of the Crayfish and that of the closely related Lobster (seeFig. 8, p. 28) is not so much that the changes in structure which occur after hatching are less profound in the former case, but that there is no free larval stage. In the Lobster the earlier stages are capable of independent existence, and they differ from the full-grown animal not only in structure, but also in habits, swimming at the surface instead of creeping at the bottom of the sea.
A similar case to that of the Crayfishes is found in the River Crabs of tropical countries, belonging to the family Potamonidæ. These Crabs are as closely related to some marine Crabs as are the Crayfishesto the Lobsters, yet the difference in their mode of development is even more pronounced. Instead of beginning life as minute pelagic zoëæ, they leave the shelter of the mother's abdomen as perfectly-formed little Crabs (Fig. 31).
Young Specimen of an African River CrabFig. 31—Young Specimen of an African River Crab(Potamon johnstoni),taken from the Abdomen of the Mother.Much enlargedThe adult of an allied species isfigured onPlate XXIII
Fig. 31—Young Specimen of an African River Crab(Potamon johnstoni),taken from the Abdomen of the Mother.Much enlarged
The adult of an allied species isfigured onPlate XXIII
Abdominal SomiteFig. 32—Early Larval Stage of a Species of Squilla, probablyS. dubia. × 10. (After Brooks.)View larger image
Fig. 32—Early Larval Stage of a Species of Squilla, probablyS. dubia. × 10. (After Brooks.)
View larger image
Amongst the Decapoda, instances of direct development like those just described are exceptional, but in some of the other orders of the Malacostraca direct development is the rule. In the great division Peracarida, as we have already seen, the females are provided with a pouch, or marsupium (from which the name of the division is derived), in which the eggs are carried. Within this pouch the young undergo the whole of their development, and they only leave it, as a rule, when they have attained the structure of the adults. Among the more familiar representatives of this division, the Sand-hoppers (Amphipoda), the Woodlice (Isopoda), and the Opossum Shrimps (Mysidacea), may be mentionedas examples of this mode of development. The Woodlice and their immediate allies differ a little from the other members of the division in the fact that the young leave the brood-pouch with the last pair of legs still undeveloped, though in other respects they are like miniature adults.
In those Crustacea which have a direct development without free-swimming larval stages, it is sometimes possible to find traces of such stages in the early development of the embryo. This is shown most clearly, perhaps, in the Opossum Shrimps (Mysidacea). In these the embryo becomes free from the egg-membrane (or may, in a sense, be said to "hatch") at a very early stage, and lies free within the brood-pouch as a maggot-shaped body, on which three pairs of rudimentary limbs can be made out. The later development shows that these three rudiments correspond to the antennules, antennæ, and mandibles, so that the maggot-shaped embryo is, in fact, a disguised nauplius without the power of swimming or of leading an independent existence. In other cases—as, for instance, in the Crayfish, where the earlier stages are confined within the egg-membrane (or "egg-shell")—the nauplius stage, although more difficult to examine, is quite as well marked.
Of the other groups of the Malacostraca, the Syncarida and Leptostraca are hatched in nearly the adult form, but the Stomatopoda have a long seriesof larval stages. These larvæ (Fig. 32) are all distinguished by the large size of the carapace, which in some cases envelops the greater part of the body. Some Stomatopod larvæ, in the warmer seas, attain to a relatively great size, sometimes exceeding 2 inches in length, and their glass-like transparency gives them a very striking appearance.
As we have seen, it is exceptional to find a free-swimming nauplius larva among the Malacostraca, but it is the commonest larval stage in the other subclasses of Crustacea. Most of the Branchiopoda are hatched in this form (Fig. 33), and reach the adult state by a very gradual series of changes in which new somites and appendages are added in regular order from before backwards till the full number is reached. The Water-fleas (Cladocera), however, differ from most of the other Branchiopoda in having a direct development. The eggs arecarried in a brood-pouch under the back of the carapace, and in this the embryos undergo their development. In the commonDaphnia, for instance, numerous eggs or young can generally be seen through the transparent carapace (seeFig. 12, p. 37).
larval Stages of the Brine ShrimpFig. 33—Larval Stages of the Brine Shrimp(Artemia salina). (After Sars.)A, Nauplius, just hatched; B-E, later stages, showing progressive increase in number of somites and appendages. The adult form of this species is shown inFig. 55, p. 164View larger image
Fig. 33—Larval Stages of the Brine Shrimp(Artemia salina). (After Sars.)
A, Nauplius, just hatched; B-E, later stages, showing progressive increase in number of somites and appendages. The adult form of this species is shown inFig. 55, p. 164
View larger image
Many of the Ostracoda have a direct development, but in some cases the young animal, on hatching,has only the first three pairs of appendages, and is therefore regarded as a nauplius, although it possesses a bivalved shell like that of the adult, and is very unlike the nauplius larvæ of other Crustacea.
Early Nauplius Larva of a CopepodFig. 34—Early Nauplius Larva of a Copepod(Cyclops).Much enlarged.(From Lankester's "Treatise on Zoology.")a′, Antennule;a″, antenna;gn, jaw-spine of antenna;lbr, upper lip;md, mandible
Fig. 34—Early Nauplius Larva of a Copepod(Cyclops).Much enlarged.(From Lankester's "Treatise on Zoology.")
a′, Antennule;a″, antenna;gn, jaw-spine of antenna;lbr, upper lip;md, mandible
Most of the Copepoda also leave the egg in the nauplius stage; and, indeed, it was to the young of the common fresh-waterCyclops(Fig. 34)that the name ofNaupliuswas first given by the Danish naturalist, O. F. Müller, in the eighteenth century, in the belief that it was an adult and independent species of Crustacea. In the Copepoda, the changes which transform the nauplius into the adult are gradual, and consist chiefly in the successive addition of new somites and appendages.
Larval Stages of the Common Rock BarnacleFig. 35—Larval Stages of the Common Rock Barnacle(Balanus balanoides—seePlate III.)A, Nauplius stage (after Hoek); B, cypris stage (after Spence Bate)View larger image
Fig. 35—Larval Stages of the Common Rock Barnacle(Balanus balanoides—seePlate III.)
A, Nauplius stage (after Hoek); B, cypris stage (after Spence Bate)
View larger image
The development of the Cirripedia is of special interest, since itwas the discovery of the larval stages by J. Vaughan Thompson that first demonstrated to naturalists that the Barnacles were Crustacea and not, as had been supposed, Molluscs. The earliest stage is generally a nauplius (Fig. 35, A) of very peculiar and characteristic form, with a pair of horns projecting sideways from the front corners of the dorsal shield, and a forked spine on the under-sidebehind. The later development is very unlike those which have been described above, for after a series of nauplius stages the larva passes suddenly, at a single moult, into a stage in which the body and limbs are enclosed in a bivalved shell (Fig. 35, B). From the superficial resemblance of the shell to that of an Ostracod, this is known as thecyprisstage. Through the valves of the shell a pair of large compound eyes can be seen, as well as six pairs of two-branched swimming feet, while in front a pair of antennules project between the valves. On each antennule is a sucker-like disc by means of which the larva, after swimming freely for some time, attaches itself to a stone or some other object, where it remains fixed for the rest of its life. A cementing substance produced by a gland at the base of the antennules attaches the front part of the head firmly to the support; the valves of the shell are cast off, and replaced by the rudimentary valves of the adult shell; the six pairs of swimming feet grow out into tendril-like cirri; the compound eyes disappear, and the animal assumes the structure of the adult.
The parasitic Rhizocephala have a very remarkable life-history, which will be described in a later chapter; but it may be mentioned here that their free-swimming larval stages resemble very closely those of the ordinary Barnacles. It was the discovery of this fact which led to its being recognized that the Rhizocephala are highly modified anddegenerate Cirripedes, although their structure in the adult state gives little evidence of their affinities.
A number of interesting problems in speculative biology are suggested by the larval stages of Crustacea. A full discussion of these problems would involve matters too technical for these pages, but some indication of the broader issues may be attempted.
The obvious question, Why do some Crustacea pass through a complicated metamorphosis while others do not? is, like many obvious and simple questions, one of the most difficult to answer. It will be pointed out later, in dealing with the fresh-water Crustacea, that one of the most general characters of fresh-water animals as compared with their marine allies is the absence of free-swimming larval stages. This applies, for instance, to the case of the Crayfishes and the marine Lobsters, and to that of the River Crabs, as compared with those which live in the sea. But it does not apply to all fresh-water Crustacea, and, on the other hand, there are many cases of direct development in marine species.
Some of the advantages gained by the possession of free-swimming larval stages are obvious enough. Many Crustacea which live on the sea-bottom, and are not very powerful swimmers, have their progeny scattered far and wide by winds and currents while in the surface-living larval stages. In the extremecase of the Barnacles, which are fixed to one spot when adult, a locomotive larval stage is clearly a necessity. But, here as elsewhere, to demonstrate the usefulness of any character is to go only a very little way towards explaining its origin. Moreover, the mere necessity for a locomotive larva throws no light on the remarkable resemblances between the larval stages of widely different species. In the adult state, a Branchiopod, a Copepod, an Ostracod, a Barnacle, and a Penæid Prawn, are separated by enormous differences of form and structure; yet, as we have seen, all these are hatched from the egg as six-limbed nauplius larvæ differing from each other only in trivial details. It seems hardly possible to imagine any other interpretation of this very striking fact than is afforded by the theory of Evolution. We are forced to assume that all these diverse forms of Crustacea are descended from very similar or identical ancestral types, and that the modifications arising in the course of their evolution have affected the adult but not the larval stages. Some naturalists would go farther than this, and would apply the so-called "theory of recapitulation" to the larval stages of the Crustacea. According to this theory, the stages in the development of any animal tend to recapitulate, more or less closely, the history of the race. Thus it is assumed, for instance, that the nauplius reproduces the structure of a six-limbed ancestral form, from which, in the distantpast, all the diverse branches of the Crustacean class took their origin. There are, however, considerable difficulties in the way of this view. That some such ancestral type did exist may be regarded as tolerably certain; that it resembled in its adult state the nauplius larvæ of present-day Crustacea is, on the whole, unlikely; but it is not at all improbable, whatever its adult structure may have been, that it hatched from the egg as a nauplius larva.
With regard to some of the other larval forms, it is possible to speak with a little more confidence. There are good grounds for believing, apart from the evidence of development, that the Lobster and its allies have descended from Crustacea which, like the existing Euphausiacea, possessed swimming branches (exopodites) on the thoracic legs; and there seems no reason to doubt that the "schizopod" larva of the Lobster does recapitulate this stage in the evolution of the race. On the other hand, it is impossible to believe that any of the ancestors of the Shore Crab resembled, even remotely, the zoëa stage with which the life-history of the individual now begins.
The tract of seashore which is laid bare by the retreat of the tide offers on most coasts a rich collecting-ground to the student of Crustacea. In places where shelving, weed-covered rocks run out to sea, innumerable Crustacea have their home in the rock-pools, or lurk in crannies awaiting the return of the tide. On sandy beaches, at first sight apparently barren of life, a closer search will reveal a whole fauna, amongst which burrowing Crustacea of various orders are prominent. Further, the shore collector will find from time to time stray specimens of forms that have their proper habitat beyond low-tide mark, and occasionally their remains are thrown in quantities on the beach by storms. It is convenient, therefore, to treat the Crustacea of the shore as a sample of those inhabiting the shallower waters of the ocean. In these shallower waters—down to the limit where light no longer penetrates from above, where vegetable life ceases, and where the strangely modified inhabitants of the deep sea begin to appear—thesea-bottom is perhaps the most densely populated of all parts of the earth's surface. Nowhere, at all events, do we find so wide a range of animal forms, from the simplest organisms (Protozoa) up to highly-organized Vertebrates. Nowhere, perhaps, is the struggle for existence more keen, and it is not without justice that some naturalists have regarded the shallow waters of the sea as "one of the great battle-fields of life," where, in the long course of evolution, the main branches of the animal kingdom have had their origin.
Conspicuous among the animals of this region are Crustacea of all sorts and sizes. To identify all the species that may be obtained in a single haul of the dredge in British seas would sometimes be a hard task even for the most expert student of the group. Our present purpose, however, is not to compile a faunistic catalogue, but merely to give some idea of the endless diversity of form, and to note a few of the "shifts for a living"—of the ways in which structure and habit are adapted to the conditions of life in the Crustacea of the shore and of shallow water.
Though it might seem that the heavily armoured Lobsters and the larger Crabs would be sufficiently protected against most enemies when once they have attained their full size, yet they are preyed upon by the Octopus, which seizes them with its suckers and pierces their armour with its powerful beak, injectinga poison that paralyzes its victims. Some years ago a "plague" of Octopus very seriously affected the Lobster fishery in the English Channel. To escape from enemies such as these, the Lobsters and many Crabs have the habit of lurking in crevices of the rocks, while in case of sudden alarm the Lobster may escape from danger by swimming, or rather darting, with great swiftness, tail foremost, through the water by powerful strokes of the abdomen and tail-fan. In the more lightly armed Prawns and other Crustacea of the tribe Natantia, which are characteristically swimmers, the power of rapid motion is probably the chief means of protection against enemies. There is reason to believe that the Lobsters have been derived from prawn-like swimming forms which have sacrificed some of their agility in developing their heavy armour-plating, retaining, however, the power of sudden and rapid motion in emergency. This power, again, has been lost by the typical Crabs (Brachyura), in which the abdomen is reduced in size and without a tail-fan, so as to be useless for swimming. While most of the Crabs, however, are somewhat slow of movement, trusting to their armour and their powerful pincers for defence, the Swimming Crabs (Portunidæ—Plate XIII.) have reacquired the power of swimming by means of the paddle-shaped legs of the last pair. Some of the tropical species of Portunidæ are probably the most expert swimmers among the Crustacea,and are described as shooting through the water like fish.
A Common Hermit Crab removed from the ShellFig. 36—A Common Hermit Crab(Eupagurus bernhardus)removed from the ShellView larger image
Fig. 36—A Common Hermit Crab(Eupagurus bernhardus)removed from the Shell
View larger image
A Symmetrical Hermit CrabFig. 37—Pylocheles miersii,a Symmetrical Hermit Crab. (After Alcock.)The upper figure gives an end view of the animal lodged in a tube of water-logged mangrove or bamboo, its large claws closing the opening. The lower figure shows the animal removed from its shelter.View larger image
Fig. 37—Pylocheles miersii,a Symmetrical Hermit Crab. (After Alcock.)
The upper figure gives an end view of the animal lodged in a tube of water-logged mangrove or bamboo, its large claws closing the opening. The lower figure shows the animal removed from its shelter.
View larger image
The Lobster's habit of seeking shelter in rock-crevices or under stones is one which is shared by a very large number of shore Crustacea. From some primitive kind of Lobster which discovered the advantages of a portable shelter have been derived the Hermit Crabs. In rock-pools one may often see whelk or periwinkle shells tumbling about with anactivity quite foreign to the nature of their original molluscan inhabitants, and closer examination will show that each contains a Hermit Crab, which retreats into the shell when disturbed. If extracted from the shell, the Crab (Fig. 36) can be seen to be most beautifully adapted to its peculiar mode of life. The abdomen is soft and spirally twisted to fit into the interior of the spiral shell, and the uropods, instead of forming a tail-fan, are modified into holding organs, with roughened, file-like surfaces which can be pressed outwards against the walls of the shell, and wedge the body so firmly that an attempt to drag the animal forcibly from its retreat often results in tearing it in half. The front part of the body, which is exposed when the animal is walking, retains its shelly armour. One of the pincer-claws, most commonly the right, is much larger than the other, and serves to block the opening of the shell when the body is withdrawn into it. The next two pairs of legs are long and slender, and are used for walking; but the last two pairs are short, with a roughened surface at the end, and serve to steady the body in the mouth of the shell. The swimmerets on the right side of the body, which is pressed against the central pillar of the shell, have disappeared, but those of the left side remain.
As the Hermit grows, it is necessary for him to remove from time to time into a larger dwelling.It has been stated that he will sometimes dispossess the rightful owner of a whelk-shell for this purpose, dragging him out piecemeal and eating him; but other observers deny that this ever happens, and in most cases, at all events, the Hermit is content to wait until he finds an empty shell of suitable size. After turning this over and exploring the interior with his claws, to satisfy himself that it is unoccupied, he deftly whips the unprotected hinder part of his body into the new habitation, keeping hold of the old one meanwhile, so that he can return to it if the other proves unsuitable. The Hermits are very pugnacious, and fight with one another for the possession of desirable shells, the victor dragging his opponent out and establishing himself in his place. Besides appropriating the shell of a dead Mollusc, many Hermits seem to go into partnership with living animals of various kinds, and some of these associations will be noticed in a later chapter. A number of species adopt other dwellings than molluscan shells, and some tropical Hermits, for instance, are found living in the cavities of water-logged stems of bamboo (Fig. 37); while others, relinquishing the advantages of a portable shelter, live in holes in corals or in the canals of living sponges. Although in some of these cases the body is straight, it usually shows traces of its original adaptation to a spiral shell in having no swimmerets on the left side.
The only Hermits which have a full series of swimmerets are theprimitive Pylochelidæ (Fig. 37), which come very near to what we imagine the ancestral form of the group to have been like, and can hardly be separated from the mud-burrowing, lobster-like Thalassinidea. A few Hermits have given up altogether the use of any protective covering. One of these is the Coconut Crab (Birgus), to be mentioned when we come to deal with the Crustacea of the land. Another is the Stone Crab (Lithodes—Plate VIII.) of our own seas, and its kindred, which have redeveloped shelly plates on the back of the abdomen, but carry it doubled up under the body like the true Crabs.These also preserve some traces of the original twisting of the abdomen, and have swimmerets only on one side.
Some Crustacea construct habitations for themselves. On turning over a flat stone between tide-marks, one often finds a little mass of bits of weed and rubbish attached to it, and if this be torn open a greenish-brown, shrimp-like animal, about three-quarters of an inch long, is seen slithering away on its side. This is an Amphipod (Amphithoë rubricata) which builds the shelter for itself, sticking the fragments together with threads of a cementing material produced by glands on the surface of its body and legs. Other Amphipods construct more neatly finished tubular dwellings of mud, or even of small stones, which are attached to sea-weeds and the like; and some make portable shelters of the same kind, which they carry about with them like the caddis-worms of fresh-water streams.
Some of the true Crabs also employ portable shields for purposes of defence or of concealment. The species ofDorippewhich are found in tropical seas have the last two pairs of legs short, elevated on the back so that they cannot be used for walking, and ending in a kind of grasping claw. By means of these claws the Crab holds over its back some object, generally one valve of a molluscan shell, sometimes even a mangrove-leaf, to supplement the protection afforded by its carapace. The "SpongeCrabs" (Dromiidæ), of which one species,Dromia vulgaris(Plate IX.), occurs on the southern coasts of Britain, have also the last two pairs of legs elevated on the back and used in a similar way; but in this case the covering is usually a mass of living sponge, one of the Sea-squirts (Tunicata), or some similar organism.