Zoæa of Thia politaFig. 210. Zoæa of Thia polita.(After Claus.)mxp2.second maxillipede.
Fig. 210. Zoæa of Thia polita.(After Claus.)mxp2.second maxillipede.
In the majority of the Decapoda the larva leaves the egg in a form known as the Zoæa (fig. 210). This larval form is characterised by the presence of a large cephalo-thoracic shield usually armed with lateral, anterior, and dorsal spines. The caudal segments are well developed,though without appendages, and the tail, which functions in swimming,is usually forked. The six posterior thoracic segments are, on the other hand,rudimentary or non-existent. There are seven anterior pairs of appendages shewn in detail infig. 211,viz.the two pairs of antennæ (At. I.andAt. II.), neither of them used as swimming organs, the mandibles without a palp (md), well-developed maxillæ (two pairs,mx 1andmx 2), and two or sometimes (Macrura) three pairs of biramous natatory maxillipeds (mxp 1andmxp 2). Two lateral compound stalked eyes are present, together with a median Nauplius eye. The heart has in the majority of cases only one or two (Brachyura) pairs of ostia.
Illustration: TitleFig. 211. The Appendages of a Crab Zoæa.At. I.first antenna;At. II.second antenna;md.mandible (without a palp);mx. 1.first maxilla;mx. 2.second maxilla;mxp. 1.first maxilliped;mxp. 2.second maxilliped.ex. exopodite;en. endopodite.
Fig. 211. The Appendages of a Crab Zoæa.
At. I.first antenna;At. II.second antenna;md.mandible (without a palp);mx. 1.first maxilla;mx. 2.second maxilla;mxp. 1.first maxilliped;mxp. 2.second maxilliped.
ex. exopodite;en. endopodite.
The Zoæa larva, though typically developed in the Decapoda, is not always present (e.g.Astacus and Homarus), and sometimes occurs in a very modified form. It makes its appearance in an altered garb in the ontogeny of some of the other groups.
The two Malacostracan forms, amongst those so far studied, in which the phylogenetic record is most fully preserved in the ontogeny, are Euphausia amongst the Schizopods and Penæus amongst the Decapods.
Schizopoda.Euphausia leaves the egg (Metschnikoff,No.468‑9) as a true Nauplius with only three pairs of appendages, the two hinderbiramous, and an unsegmented body. The second pair of antennæ has not however the colossal dimensions so common in the lower types. A mouth is present, but the anus is undeveloped.
Nauplius of EuphausiaFig. 212. Nauplius of Euphausia.(From Claus; after Metschnikoff.)The Nauplius is represented shortly before an ecdysis, and in addition to the proper appendages rudiments of the three following pairs are present.OL.upper lip;UL.lower lip;Md.mandible;Mx´.andMx´´.two pairs of maxillæ;mf´.maxilliped 1.
Fig. 212. Nauplius of Euphausia.(From Claus; after Metschnikoff.)
The Nauplius is represented shortly before an ecdysis, and in addition to the proper appendages rudiments of the three following pairs are present.OL.upper lip;UL.lower lip;Md.mandible;Mx´.andMx´´.two pairs of maxillæ;mf´.maxilliped 1.
After the first moult three pairs of prominences—the rudiments of the two maxillæ and 1st maxillipeds arise behind the Nauplius appendages (fig. 212). At the same time an anus appears between the two limbs of a rudimentary caudal fork; and an unpaired eye and upper lip appear in front. After another moult (fig. 212) a lower lip is formed (UL) as a pair of prominences very similar to true appendages; and a delicate cephalo-thoracic shield also becomes developed. Still later the cutting blade of the mandible is formed, and the palp (Nauplius appendage) is greatly reduced. The cephalo-thoracic shield grows over the front part of the embryo, and becomes characteristically toothed at its edge. There are alsotwo frontal papillæ very similar to those already described in the Phyllopod larvæ. Rudiments of the compound eyes make their appearance, and though no new appendages are added, those already present undergo further differentiations. They remain however very simple; the maxillipeds especially are very short and resemble somewhat Phyllopod appendages.
Up to this stage the tail has remained rudimentary and short, but after a further ecdysis (Claus) it grows greatly in length. At the same time the cephalo-thoracic shield acquires a short spine directed backwards. The larva is now in a condition to which Claus has given the name of Protozoæa (fig. 213A).
Very shortly afterwards the region immediately following the segments already formed becomes indistinctly segmented, while the tail is still without a trace of segmentation. The region of the thorax proper soon becomes distinctly divided into seven very short segments, while at the same time the now elongated caudal region has become divided into its normal number of segments (fig. 213B). By this stage the larva has become a true Zoæa—though differing from the normal Zoæa in the fact that the thoracic region is segmented, and in the absence of a second pair of maxillipeds.
Larvæ of EuphausiaFig. 213. Larvæ of Euphausia.(After Claus.) From the side.A. Protozoæa larva. B. Zoæa larva.mx.´andmx´´.maxillæ 1 and 2;mxp1.maxilliped 1.
Fig. 213. Larvæ of Euphausia.(After Claus.) From the side.
A. Protozoæa larva. B. Zoæa larva.mx.´andmx´´.maxillæ 1 and 2;mxp1.maxilliped 1.
The adult characters are very gradually acquired in a series of successive moults; the later development of Euphausia resembling in this respect that of the Phyllopods. On the other hand Euphausia differs from that group in the fact that the abdominal (caudal) and thoracic appendages develop astwo independent seriesfrom before backwards, of which the abdominal series is the earliest to attain maturity.
This is shewn in the following table compiled from Claus’ observations.
All the appendages following the second pair of maxillæ are biramous, and the first eight of these bear branched gills as their epipodites. It is remarkable that the epipodite is developed on all the appendages anteriorly in point of time to the outer ramus (exopodite).
Although in Mysis there is no free larval stage, and the development takes place in a maternal incubatory pouch, yet a stage may be detected which clearly corresponds with the Nauplius stage of Euphausia (E. van Beneden,No.465). At this stage, in which only the three Nauplius appendages are developed, the Mysis embryo is hatched. An ecdysis takes place, but the Nauplius skin is not completely thrown off, and remains as an envelope surrounding the larva during its later development.
Decapoda.Amongst the Decapoda the larva usually leaves the egg in the Zoæa form, but a remarkable exception to this general rule is afforded by the case of one or more species of Penæus. Fritz Müller was the first to shew that the larva of these forms leaves the egg as atypical Nauplius, and it is probable that in the successive larval stages of these forms the ancestral history of the Decapoda is most fully preserved[188].
The youngest known larva of Penæus (fig. 214) has a somewhat oval unsegmented body. There spring from it the three typical pairs of Nauplius appendages. The first is uniramous, the second and third are biramous, and both of them adaptedfor swimming, and the third of them (mandibles) is without a trace of the future blade. The body has no carapace, and bears anteriorly a single median simple eye. Posteriorly it is produced into two bristles.
Nauplius stage of PenæusFig. 214. Nauplius stage of Penæus.(After Fritz Müller.)
Fig. 214. Nauplius stage of Penæus.(After Fritz Müller.)
After the first moult the larva has a rudiment of a forked tail, while a dorsal fold of skin indicates the commencement of the cephalo-thoracic shield. A large provisional helmet-shaped upper lip like that in Phyllopods has also appeared. Behind the appendages already formed there are stump-like rudiments of the four succeeding pairs (two pairs of maxillæ and two pairs of maxillipeds); and in a slightly older larva the formation of the mandibular blade has commenced, together with the atrophy of the palp or Nauplius appendage.
Protozoæa stage of PenæusFig. 215. Protozoæa stage of Penæus.(After Fritz Müller.)
Fig. 215. Protozoæa stage of Penæus.(After Fritz Müller.)
Between this and the next observed stage there is possibly a slight lacuna. The next stage (fig. 215) at any rate represents the commencement of the Zoæa series. The cephalo-thoracic shield has greatly grown, and eventually acquires the usual dorsal spine. The posterior region of the body is prolonged into a tail, which is quite as long as the whole of the remainder of the body. The four appendages which were quite functionless at the last stage have now sprouted into full activity. Theregion immediately behind them is divided (fig. 215) into six segments (the six thoracic segments) without appendages, while somewhat later the five anterior abdominal segments become indicated, but are equally with the thoracic segments without feet. The mode of appearance of these segments shews that the thoracic and abdominal segments develop in regular succession from before backwards (Claus). Of the palp of the mandibles, as is usual amongst Zoæa forms, not a trace remains, though in the youngest Zoæa caught by Fritz Müller a very small rudiment of the palp was present. The first pair of antennæ is unusually long, and the second pair continues to function as a biramous swimming organ; the outer ramus is multiarticulate. The other appendages are fully jointed, and the two maxillipeds biramous. On the dorsal surface of the body the unpaired eye is still present, but on each side of it traces of the stalked eyes have appeared. Frontal sense organs like those of Phyllopods are also present.
From the Protozoæa form the larva passes into that of a true Zoæa with the usual appendages and spines, characterised however by certain remarkable peculiarities. Of these the most important are (1) the large size of the two pairs of antennæ and the retention of its Nauplius function by the second of them; (2) the fact that the appendages of the six thoracic segments appear as small biramous Schizopod legs, while the abdominal appendages, with the exception of the sixth, are still withouttheir swimming feet. The early appearance of the appendages of the sixth abdominal segment is probably correlated with their natatory function in connection with the tail. As a point of smaller importance which may be mentioned is the fact that both pairs of maxillæ are provided with small respiratory plates (exopodites) for regulating the flow of water under the dorsal shield. From the Zoæa form the larva passes into a Mysis or Schizopod stage (fig. 216), characterised by the thoracic feet and maxillipeds resembling in form and function the biramous feet of Mysis, the outer ramus being at first in many cases much larger than the inner. The gill pouches appear at the base of these feet nearly at the same time as the endopodites become functional. At the same time the antennæ become profoundly modified. The anterior antennæ shed their long hairs, and from the inner side of the fourth joint there springs a new process, which eventually elongates and becomes the inner flagellum. The outer ramus of the posterior antennæ is reduced to a scale, while the flagellum is developed from a stump-like rudiment of the inner ramus (Claus). A palp sprouts on the mandible and the median eye disappears.
Penæus larva in the Mysis stageFig. 216. Penæus larva in the Mysis stage.(After Claus.)
Fig. 216. Penæus larva in the Mysis stage.(After Claus.)
The abdominal feet do not appear till the commencement of the Mysis stage, and hardly become functional till its close.
From the Mysis stage the larva passes quite simply into the adult form. The outer ramus of the thoracic feet is more or less completely lost. The maxillipeds, or the two anterior pairs at any rate, lose their ambulatory function, cutting plates develop on the inner side of their basal joints, and the two rami persistas small appendages on their outer side. Gill pouches also sprout from their outer side.
The respiratory plate of the second maxilla attains its full development and that on the first maxilla disappears[189]. The Nauplius, so far as is known, does not occur in any other Decapod form except Penæus.
Protozoæa stage of Sergestes larvaFig. 217. Latest Protozoæa stage of Sergestes larva (Elaphocaris).(After Claus.)mxp´´´.third pair of maxillipeds.
Fig. 217. Latest Protozoæa stage of Sergestes larva (Elaphocaris).(After Claus.)
mxp´´´.third pair of maxillipeds.
The next most primitive larval history known is that which appears in the Sergestidæ. The larval history, which has been fully elucidated by Claus, commences with a Protozoæa form (fig. 217), which develops into a remarkable Zoæa first described by Dohrn as Elaphocaris. This develops into a form originally described by Claus as Acanthosoma, and this into a form known as Mastigopus (fig. 218) from which it is easy to pass to the adult.
The remarkable Protozoæa (fig. 217) is characterised by the presence on the dorsal shield of a frontal, dorsal and two lateral spikes, each richly armed with long side spines. The normal Zoæa appendages are present, and in addition to them a small third pair of maxillipeds. The thoracic region is divided into five short rings, but the abdomen is unsegmented. The tail is forked and provided with long spines. The antennæ, like those of Penæus, are long—the second pair biramous; the mandibles unpalped. Both pairs of maxillæ are provided with respiratory plates; the second pair is footlike, and has at its base a glandular mass believed by Claus to be the equivalent of the Entomostracan shell-gland. The maxillipeds have the usual biramous characters. Ahelmet-shaped upper lip like that of a typical Nauplius is present, and the eyes are situated on very long stalks.
Mastigopus Stage Of SergestesFig. 218. Mastigopus Stage Of Sergestes.(From Claus.)Mf´´´.maxilliped 3.
Fig. 218. Mastigopus Stage Of Sergestes.(From Claus.)
Mf´´´.maxilliped 3.
In the true Zoæa stage there appear on the five thoracic segments pouch-like biramous rudiments of the limbs. The tail becomes segmented; but the segments, with the exception of the sixth, remain without appendages. On the sixth a very long bilobed pouch appears as the commencement of the swimming feet of this segment. The segments of the abdomen are armed with lateral spines.
Larva of HippolyteFig. 219. Larva of Hippolyte In Zoæa Stage.(From Claus.)Mx´.andMx´´.maxillæ 1 and 2;Mf´. Mf´´. Mf´´´.maxillipeds.
Fig. 219. Larva of Hippolyte In Zoæa Stage.(From Claus.)
Mx´.andMx´´.maxillæ 1 and 2;Mf´. Mf´´. Mf´´´.maxillipeds.
From the Zoæa stage the larva passes into the form known as Acanthosoma, which represents the Mysis stage of Penæus. The complex spikes on the dorsal shield of the Zoæa stage are reduced to simple spines, but the spines of the tail still retain their full size. In the appendages the chief changes consist (1) in the reduction of the jointed outer ramus of the second pair of antennæ to a stump representing the scale, and the elongation of the inner one to the flagellum; (2) in the elongation of the five ambulatory thoracic appendages into biramous feet, like the maxillipeds, and in the sprouting forth of rudimentary abdominal feet.
The most obvious external indications of the passage from the Acanthosoma to the Mastigopus stage (fig. 218) are to be found in the elongation of the abdomen, the reduction and flattening of the cephalo-thoracic shield, and the nearly complete obliteration of all the spines but the anterior. The eyes on their elongated stalks are still very characteristic, and the elongation of the flagellum of the second pair of antennæ is very striking.
Older Larva of HippolyteFig. 220. Older Larva of Hippolyte after the thoracic appendages have become formed.(From Claus.)
Fig. 220. Older Larva of Hippolyte after the thoracic appendages have become formed.(From Claus.)
The maxillæ and maxillipeds undergo considerable metamorphosis, the abdominal feet attain their adult form, and the three anterior thoracic ambulatory legs lose their outer rami. The most remarkable change of all concerns the two last pairs of thoracic appendages, which, instead of being metamorphosed like the preceding ones, are completely or nearly completely thrown off in the moult which inaugurates the Mastigopus stage, and are subsequently redeveloped. With the reappearance of these appendages, and the changes in the other appendages already indicated, the adult form is practically attained.
With reference to the development of the majority of the Carabidæ, Penæinæ, Palæmoninæ, Crangoninæ, it may be stated generally that they leave the egg in the Zoæa stage (fig. 219) with anterior appendages up to the third pair of maxillipeds. The thorax is unsegmented and indeed almost unrepresented, but the abdomen is long and divided into distinct segments. Both thoracic and abdominal appendages are absent, and the tail is formed by a simple plate with numerous bristles, not forked, as in the case of the Zoæa of Fritz Müller’s Penæus and Sergestes. A dorsal spine is frequently found on the second abdominal segment. From the Zoæa form the embryo passes into a Mysis stage (fig. 220), during which the thoracic appendages gradually appear as biramous swimming feet; they are all developed before any of the abdominal appendages, except the last. In some cases the development is still further abbreviated. Thus the larvæ of Crangon and Palæmonetes (Faxon,No.476) possess at hatching the rudiments of the two anterior pairs of thoracic feet, and Palæmon of three pairs[190].
Newly-hatched Larva of the American LobsterFig. 221. Newly-hatched Larva of the American Lobster.(After Smith.)
Fig. 221. Newly-hatched Larva of the American Lobster.(After Smith.)
Amongst the other Macrura the larva generally leaves the egg as a Zoæa similar to that of the prawns. In the case of theThalassinidæ and Paguridæ a Mysis stage has disappeared. The most remarkable abbreviations of the typical development are presented on the one hand by Homarus and Astacus, and on the other by the Loricata.
The development of Homarus has been fully worked out by S. J. Smith (No.491) for the American lobster (Homarus americanus). The larva (fig. 221) leaves the egg in an advanced Mysis stage. The cephalo-thoracic shield is fully developed, and armed with a rostrum in front. The first pair of antennæ is unjointed but the second is biramous, the outer ramus forming a large Mysis-like scale. The mandibles, which are palped, the maxillæ, and the two anterior maxillipeds differ only in minor details from the same appendages of the adult. The third pair of maxillipeds is Mysis-like and biramous, and the five ambulatory legs closely resemble them, the endopodite of the first being imperfectly chelate. The abdomen is well developed but without appendages. The second, third, fourth and fifth segments are armed with dorsal and lateral spines.
In the next stage swimming feet have appeared on the second, third, fourth and fifth abdominal segments, and the appendages already present have approached their adult form. Still later, when the larva is about half an inch in length, the approach to the adult form is more marked, and the exopodites of the ambulatory legs though present are relatively much reduced in size. The swimmerets of the sixth abdominal segment are formed. In the next stage observed the larva has entirely lost its Schizopod characters, and though still retaining its free-swimming habits differs from the adult form only in generic characters.
As has been already stated, no free larval stages occur in the development of Astacus, but the young is hatched in a form in which it differs only in unimportant details from the adult.
The peculiar larval form of the Loricata (Scyllarus, Palinurus) has long been known under the name Phyllosoma (fig. 222C), but its true nature was first shewn by Couch (No.474) [Couch did not however recognise the identity of his larva with Phyllosoma; this was first done by Gerstäcker] and shortly afterwards by Gerbe and Coste. These observations were however for a long time not generally accepted, till Dohrn (No.477) published his valuable memoir giving an account of how he succeeded in actually rearing Phyllosoma from the eggs of Scyllarus and Palinurus, and shewing that some of the most remarkable features of the metamorphosis of the Loricata occur before the larva is hatched.
The embryo of Scyllarus in the egg first of all passes through the usual Nauplius stage, and then after the formation of a cuticle develops an elongated thoracico-abdominal region bent completely over the anterior part of the body. There appear moreover a number of appendages and the rudiments of various organs; and the embryo passes into a form which may be described as the embryonic Phyllosoma stage. In this stage there are present on the anterior part of the body, in front of the ventral flexure, twopairs of antennæ, mandibles, two pairs of maxillæ, the second commencing to be biramous,and a small stump representing the first pair of maxillipeds. The part of the body bent over consists of a small quadrate caudal plate, and an appendage-bearing region to which are attached anteriorly three pairs of biramous appendages—the second and third maxillipeds, and the anterior pair of ambulatory legs—and two pairs of undivided appendages—the second and third pairs of ambulatory legs. In a slightly later stage the first pair of maxillæ becomes biramous, as also does the first pair of maxillipeds in a very rudimentary fashion. The second and third pairs of ambulatory legs become biramous, while the second and third maxilliped nearly completely lose their outer ramus. Very small rudiments of the two hinder ambulatory legs become formed. If the embryo is taken at this stage (videfig. 222A, which represents a nearly similar larva of Palinurus) out of the egg, it is seen to consist of (1) an anterior enlargement with a vaulted dorsal shield enclosing the yolk, two stalked eyes, and a median eye; (2) a thoracic region in which the indications of segmentation are visible with the two posterior pairs of maxillipeds (mxp2andmxp3) and the ambulatory legs (p1); (3) an abdominal region distinctly divided into segments and ending in a fork.
Larvæ of the LoricataFig. 222. Larvæ of the Loricata.(After Claus.)A. Embryo of Palinurus shortly before hatching.B. Young Phyllosoma larva of Scyllarus, without the first maxilliped, the two last thoracic appendages, or the abdominal appendages.C. Fully-grown Phyllosoma with all the Decapod appendages.at1.antenna of first pair;at2.antenna of second pair;md.mandible;mx1.first maxilla;mx2.second maxilla;mxp1-mxp3.maxillipeds;p1-p3.thoracic appendages.
Fig. 222. Larvæ of the Loricata.(After Claus.)
A. Embryo of Palinurus shortly before hatching.B. Young Phyllosoma larva of Scyllarus, without the first maxilliped, the two last thoracic appendages, or the abdominal appendages.C. Fully-grown Phyllosoma with all the Decapod appendages.
at1.antenna of first pair;at2.antenna of second pair;md.mandible;mx1.first maxilla;mx2.second maxilla;mxp1-mxp3.maxillipeds;p1-p3.thoracic appendages.
Before the embryo becomes hatchedthe first pair of maxillipeds becomes reduced in size and finally vanishes. The second pair of maxillæ becomes reduced to simple stumps with a few bristles, the second pair of antennæalso appears to undergo a retrogressive change, while the two last thoracic segments cease to be distinguishable. It thus appears that during embryonic life the second pair of antennæ, the second pair of maxillæ, and the second and third pair of maxillipeds and the two hinder ambulatory appendages undergo retrogressive changes, while the first pair of maxillipeds is completely obliterated!
The general form of the larva when hatched (fig. 222B) is not very different from that which it has during the later stages within the egg. The body is divided into three regions: (1) an anterior cephalic, (2) a middle thoracic, and (3) a small posterior abdominal portion; and all of them are characterised by their extreme dorso-ventral compression, so that the whole animal has the form of a three-lobed disc, the strange appearance of which is much increased by its glass-like transparency.
The cephalic portion is oval and projects slightly behind so as to overlap the thorax. Its upper surface constitutes the dorsal shield, from which there spring anteriorly the two compound eyes on long stalks, between which is a median Nauplius eye. The mouth is situated about the middle of the under surface of the anterior disc. It leads into a stomach from which an anterior and a lateral hepatic diverticulum springs out on each side. The former remains as a simple diverticulum through larval life, but the latter becomes an extremely complicated glandular structure.
At the front border of the disc is placed the unjointed but elongated first pair of antennæ (at1). Externally to and behind these there spring the short posterior antennæ (at2), at the base of which the green gland is already formed. Surrounding the mouth are the mandibles (md) and anterior pair of maxillæ (mx1), and some distance behind the second pair of maxillæ (mx2), consisting of a cylindrical basal joint and short terminal joint armed with bristles. The first pair of maxillipeds is absent.
The thoracic region is formed of an oval segmented disc attached to the under surface of the cephalic disc. From its front segment arises the second pair of maxillipeds (mxp2) as single five-jointed appendages, and from the next segment springs the five-jointed elongated but uniramous third pair of maxillipeds (mxp3), and behind this there arise three pairs of six-jointed ambulatory appendages (p1, p2, p3, of which only the basal joint is represented in the figure) with an exopodite springing from their second joint. The two posterior thoracic rings and their appendages cannot be made out.
The abdomen is reduced to a short imperfectly segmented stump, ending in a fork, between the prongs of which the anus opens. Even the youngest larval Phyllosoma, such as has just been described, cannot be compared with a Zoæa, but belongs rather, in the possession of biramous thoracic feet, to a Mysis stage. In the forked tail and Nauplius eye there appear however to be certain very primitive characters carried on to this stage.
The passage of this young larva to the fully formed Phyllosoma (fig. 222C) is very simple. It consists essentially in the fresh development of the first pair of maxillipeds and the two last ambulatory appendages, the growth and segmentation of the abdomen, and the sprouting on it of biramousswimming feet. In the course of these changes the larva becomes a true Decapod in the arrangement and number of its appendages; and indeed it was united with this group before its larval character was made out. In addition to the appearance of new appendages certain changes take place in those already present. The two posterior maxillipeds, in the Palinurus Phyllosoma at any rate, acquire again an exopodite, and together with the biramous ambulatory feet develop epipodites in the form of gill pouches.
The mode of passage of the Phyllosoma to the adult is not known, but it can easily be seen from the oldest Phyllosoma forms that the dorsal cephalic plate grows over the thorax, and gives rise to the cephalo-thoracic shield of the adult.
There are slight structural differences, especially in the antennæ, between the Phyllosoma of Scyllarus and that of Palinurus, but the chief difference in development is that the first pair of maxillipeds of the Palinurus embryo, though reduced in the embryonic state, does not completely vanish, at any rate till after the free larval state has commenced; and it is doubtful if it does so even then. The freshly hatched Palinurus Phyllosoma is very considerably more developed than that of Scyllarus.
The Appendages of a Crab ZoæaFig. 223. The Appendages of a Crab Zoæa.At. I.first antenna;At. II.second antenna;md.mandible (without a palp);mx. 1.first maxilla;mx. 2.second maxilla;mx. 3.third maxilla;mxp. 1.first maxilliped;mxp. 2.second maxilliped.ex.exopodite;en.endopodite.
Fig. 223. The Appendages of a Crab Zoæa.
At. I.first antenna;At. II.second antenna;md.mandible (without a palp);mx. 1.first maxilla;mx. 2.second maxilla;mx. 3.third maxilla;mxp. 1.first maxilliped;mxp. 2.second maxilliped.
ex.exopodite;en.endopodite.
Brachyura.All the Brachyura, with the exception of one or more species of land crabs[191], leave the egg in the Zoæa condition, and though there are slight variations of structure, yet on the whole the Crab Zoæa is a very well marked form. Immediately after leaving the egg (fig. 210) it has a somewhat oval shapewith a long distinctly segmented abdomen bent underneath the thorax. The cephalo-thoracic shield covers over the front part of the body, and is prolonged into a long frontal spine pointing forwards, and springing from the region between the two eyes; a long dorsal spine pointing backwards; and two lateral spines.
To the under surface of the body are attached the anterior appendages up to the second maxilliped, while the six following pairs of thoracic appendages are either absent or represented only in a very rudimentary form. The abdomen is without appendages.
Illustration: TitleFig. 224. Crab Zoæa after the third pair of maxillipeds and the thoracic and abdominal appendages have become developed.at1.antenna of first pair;at2.antenna of second pair;mx1.first maxilla;mx2.second maxilla;mxp1.first maxilliped;mxp2.second maxilliped;mxp3.third maxilliped;oc.eye;ht.heart.
Fig. 224. Crab Zoæa after the third pair of maxillipeds and the thoracic and abdominal appendages have become developed.
at1.antenna of first pair;at2.antenna of second pair;mx1.first maxilla;mx2.second maxilla;mxp1.first maxilliped;mxp2.second maxilliped;mxp3.third maxilliped;oc.eye;ht.heart.
The anterior antennæ are single and unjointed, but provided at their extremity with a few olfactory hairs (only two in Carcinus Mœnas) and one or two bristles. The rudiment of the secondary flagellum appears in very young Zoææ on the inner side of the antennules (fig. 223At. I.). The posterior antennæ are without the flagellum, but are provided with a scale representing the exopodite (fig. 223At. II. ex) and usually a spinous process. The flagellum is very early developed and is represented infig. 223,At. II. en. The mandibles (md) are large but without a palp. The anterior maxillæ (mx 1) have a short two-jointed endopodite (palp) with a few hairs, and a basal portionwith two blades, of which the distal is the largest, both armed with stiff bristles. The posterior maxillæ have a small respiratory plate (exopodite), an endopodite (palp) shaped like a double blade, and two basal joints each continued into a double blade. The two maxillipeds (mxp 1andmxp 2) have the form and function of biramous swimming feet. The exopodite of both is two-jointed and bears long bristles at its extremity; the endopodite of the anterior is five-jointed and long, that of the second is three-jointed and comparatively short.
In the six-jointed tail the second segment has usually two dorsally directed spines, and the three succeeding segments each of them two posteriorly directed. The telson or swimming plate is not at first separated from the sixth segment; on each side it is prolonged into two well-marked prongs; and to each prong three bristles are usually attached (fig. 224). The heart (fig. 224ht) lies under the dorsal spine and is prolonged into an anterior, posterior, and dorsal aorta. It has only two pairs of venous ostia.
During the Zoæa stage the larva rapidly grows in size, and undergoes considerable changes in its appendages which reach the full Decapod number (fig. 224). On both pairs of antennæ a flagellum becomes developed and grows considerably in length. Before the close of the Zoæa condition a small and unjointed palp appears on the mandible. Behind the second maxilliped the third maxilliped (mxp3) early appears as a small biramous appendage, and the five ambulatory feet become distinctly formed as uniramous appendages—the exopodites not being present. The third pair of maxillipeds and three following ambulatory appendages develop gill pouches. The abdominal feet are formed on the second to the sixth segments of the tail as simple pouches.
The oldest Zoæa is transmuted at its moult into a form known as Megalopa, which is really almost identical with an anomurous Decapod. No Schizopod stage is intercalated, which shews that the development is in many respects greatly abbreviated. The essential characters of the Megalopa are to be found in (1) the reduction of the two anterior maxillipeds, which cease to function as swimming feet, and together with the appendages in front of them assume the adult form; (2) the fullfunctional development of the five ambulatory appendages; (3) the reduction of the forked telson to an oval swimming plate, and the growth in size of the abdominal feet, which become large swimming plates and are at the same time provided with short endopodites which serve to lock the feet of the two sides.
With these essential characters the form of the Megalopa differs considerably in different cases. In some instances (e.g.Carcinus mœnas) the Zoæa spines of the youngest Megalopa are so large that the larva appears almost more like a Zoæa than a Megalopa (Spence Bate,No.470). In other cases,e.g.that represented onfig. 225, the Zoæa spines are still present but much reduced; and the cephalo-thoracic shield has very much the adult form. In other cases again (e.g.Portunus) the Zoæa spines are completely thrown off at the youngest Megalopa stage.
There is a gradual passage from the youngest Megalopa to the adult form by a series of moults.
Megalopa stage of Crab larvaFig. 225. Megalopa stage of Crab larva.
Fig. 225. Megalopa stage of Crab larva.
Some of the brachyurous Zoæa forms exhibit considerable divergences from the described type, more especially in the armature of the shield. In some forms the spines are altogether absent,e.g.Maja (Couch,No.474) and Eurynome. In other forms the frontal spine may be much reduced or absent (Inachus and Achæus). The dorsal spine may also be absent, and in one form described by Dohrn (No.478) there is a long frontal spine and two pairs of lateral spines, but no dorsal spine. Both dorsal and frontal spines may attain enormous dimensions and be swollen at their extremities (Dohrn). A form has been described by Claus as Pterocaris in which the cephalo-thoracic shield is laterally expanded into two wing-like processes.
The Zoæa of Porcellana presents on the whole the most remarkable peculiarities and, as might be anticipated from the systematic position of the adult, is in some respects intermediate between the macrurous Zoæa and that of the Brachyura. It is characterized by the oval form of the body, and bythe presence of one enormously long frontal spine and two posterior spines. The usual dorsal spine is absent. The tail plate is rounded and has the character of the tail of a macrurous Zoæa, but in the young Zoæa the third pair of maxillipeds is absent and the appendages generally have a brachyurous character. A Megalopa stage is hardly represented, since the adult may almost be regarded as a permanent Megalopa.
Stomatopoda.The history of the larval forms of the Stomatopoda (Squilla etc.) has not unfortunately been thoroughly worked out, but what is known from the researches of Fritz Müller (No.495) and Claus (No.494) is of very great importance. There are it appears two types, both of which used to be described as adult forms under the respective names Erichthus and Alima.