With Chaetae Paired.
There are of course other points by which the different species can be distinguished. Colour in a few cases enables a species to be named at once without any further aid. One of the most striking of these cases is the Brandling, so common upon dunghills, and so dear to some anglers. This worm is ringed with brownish purple upon a yellowish ground. The greenishA. chloroticais often found under stones, and curls itself round into nearly a complete circle when disturbed.A. cyanea, of a bluish grey colour, is one of the earthworms very commonly met with in the early morning in London and the neighbourhood. More generally, however, the colour is of a paler or darker red, verging towards and attaining brown, or even blackish brown; and is so variable that nothing in the way of identification can be attempted from the colour alone, even with the most elaborate description.
Lumbricus, as already mentioned, is distinguished from allAllolobophoraexceptA. eiseni, by the complete dovetailing of the prostomium into the first segment. There are five species in this country which may be thus distinguished by the position of the tubercula pubertatis. The most familiar species is the commonL. rubellus.
HIRUDINEA (LEECHES)
INTRODUCTION—ANATOMY—REPRODUCTION—CLASSIFICATION—RHYNCHOBDELLAE AND GNATHOBDELLAE
"The external appearance of the Hirudinea," remarks Professor Vaillant,[450]"permits us, save for rare exceptions, to recognise at once the animals which belong to that group." The leeches are distinguished as a rule by the possession of two suckers, one at each end of the body; their general shape usually differs from that of other Annelids by its oval contour and its dorso-ventral flattening.Cyclicobdella lumbricoidesof Grube, which Blanchard has stated to be the same species asNephelis tergestina, has, however, almost the form of an earthworm by reason of its cylindrical shape and the inconspicuousness of the suckers, whileLumbricobdellaalso resembles an earthworm and has no posterior sucker at all.[451]The Oligochaet family Discodrilidae (see p.376) agree with the leeches in their parasitism, in their general shape, in the presence of two suckers, and, furthermore, in the existence of jaws, which are found in no other Oligochaet, but occur in a large number of the Hirudinea. These facts, indeed, though not perhaps important by themselves, are indications of the really close resemblance of the Hirudinea to the Oligochaeta, a group which they approach not merely in such habits as the formation of a cocoon in which the eggs are enclosed, but also in many important points of internal and external structure. Indeed, the fundamental differences between the two groups are not numerous, and are not of such importance as has been given them by some writers.
Leeches are to be found in most parts of the world, insituations which are sufficiently damp for their comfort. But we do not at present possess enough knowledge to state much as to the facts of their distribution. The structure of leeches is not so well known as is that of the earthworms; for they have not been to so great an extent collected in extra-European countries. It would even be desirable to ascertain precisely the species which inhabit these islands, the most recent enumeration (1865) being that contained in the British Museum Catalogue of non-parasitical worms by the late Dr. George Johnston. For Italy this has been lately done by Dr. Blanchard, and a good many of the species are common to the two countries. Johnston enumerates altogether (after subtracting what are probably synonyms) twenty-one species, distributed among the generaBranchellion,Pontobdella,Piscicola,Nephelis,Trocheta,Haemopis,Hirudo, andGlossiphonia(=Clepsine), which number will be possibly still further reduced. The first two genera are marine, the remainder being fresh water or terrestrial;Trochetahas been probably introduced.
fig201Fig. 201.—Anterior end ofMacrobdella sestertia, to show eyes and sense bodies. (After Whitman.)
Fig. 201.—Anterior end ofMacrobdella sestertia, to show eyes and sense bodies. (After Whitman.)
Fig. 201.—Anterior end ofMacrobdella sestertia, to show eyes and sense bodies. (After Whitman.)
The use ofHirudo medicinalisis well known to many of us from personal experience. So extensively was this leech formerly made use of that it is now far from being a common species either in this country or in France. Those who desire full information as to Hirudiniculture should consult the work of Dr. Ebrard, published in 1857.[452]The former extensive use of the leech has led to the transfer of its name to the doctor who employs it, the authors of the sixteenth century constantly terming a physician a leech; it has been suggested, however, that the term was applied rather by way of analogy. The useful blood-sucking habits of the medicinal leech have been wrongly attributed to the innocent horseleech (Aulastomum)—innocent, that is to say, of the blood of Vertebrates, for it has been described as "a cruel and greedy worm," engulfing earthworms and even smaller specimens of its own species."Horsleches," said an old writer, "are wholesome to drawe foorthe foule blood, if thei are put into a hollowe rede, and one of their endes cutte of, whereby the blood maie run forthe." But it is clearly not easy for a creature destitute of jaws and teeth to bite, and the similarity of general aspect has doubtless led to a confusion with the savagely biting medicinal leech.
fig202Fig. 202.—Sense body ofMacrobdella sestertia. (After Whitman.)ep, Epidermis;s, clear cells. Highly magnified.
Fig. 202.—Sense body ofMacrobdella sestertia. (After Whitman.)ep, Epidermis;s, clear cells. Highly magnified.
Fig. 202.—Sense body ofMacrobdella sestertia. (After Whitman.)ep, Epidermis;s, clear cells. Highly magnified.
fig203Fig. 203.—Section through eye ofHaemadipsa japonica. (After Whitman.)ep, Epidermis;n, nerve. Highly magnified.
Fig. 203.—Section through eye ofHaemadipsa japonica. (After Whitman.)ep, Epidermis;n, nerve. Highly magnified.
Fig. 203.—Section through eye ofHaemadipsa japonica. (After Whitman.)ep, Epidermis;n, nerve. Highly magnified.
The Hirudinea are all distinctly segmented animals, but the segmentation differs from that of the Oligochaeta in two points. In the first place the number of segments is much smaller in a leech than in an Oligochaete, although the difference does not appear great at first sight.
A leech's body may seem to be composed of seventy, eighty, or one hundred segments, a number quite as great as is found, for example, in the genusPerichaetaamong the earthworms; but the apparent number of segments in the leech is produced by a very marked annulation of the real segments; and this is indeed the second point of difference referred to above. But there are earthworms which show frequently a secondary annulation,—secondary because it appears late and does not affect other organsof the body. A segment of an earthworm may indeed have five or six distinct annulations, but it will be bounded internally by two septa, and will bear only one set of chaetae externally. In the leech external clues to the definition of a segment were until recently wanting. They appear now to have been found in the sensory organs of the skin (Figs. 201 and 202), which are, according to Whitman,[453]disposed in a perfectly metameric fashion. Judged by this, and also by the nephridia and nerve-ganglia, the number of segments in a leech does not appear to exceed twenty-six, independently of the sucker, which may represent a few fused segments, seven (in the medicinal leech) according to Leuckart.
The eyes, which are so useful in the systematic arrangement of the group, appear to have been evolved from these sensory organs by a further exaggeration of their peculiarities. Figs. 202 and 203 show this point convincingly. The segmental sense organ is shown in Fig. 202; to the outside of certain sense cells, below which are a mass of ganglion cells, are certain peculiar transparent cells very similar to the clear cells found in the interior of the eye (Fig. 203). The segmental disposal of the sensory bodies and of the eyes is shown in Fig. 201.
fig204Fig. 204.—Branchellion torpedinis.(From the "Règne Animal.") ×1.
Fig. 204.—Branchellion torpedinis.(From the "Règne Animal.") ×1.
Fig. 204.—Branchellion torpedinis.(From the "Règne Animal.") ×1.
Some Hirudinea are furnished with external branchiae; this is the case withBranchellion, in which genus the branchiae (Fig. 204) have an arborescent form; inCystibranchusthere are a series of paired simple vesicles which take the place of these more complicated respiratory organs ofBranchellion. The Hirudinea do not, save for one exception (Acanthobdella), possess chaetae; but it must be borne in mind that the Discodrilidae and the genusAnachaetaamong the Oligochaeta are in the same condition. InAcanthobdella[454]there are two pairs of chaetae upon each side of the anterior five segments of the body. According to the figure which Grube, theoriginal describer of the genus, gives of these chaetae, the part implanted in the body is straight, while the part extending freely beyond the body is sharply hooked.
The body of the leeches is never ciliated externally; there is, as in the higher Oligochaeta, a cuticle secreted by the underlying epidermis. The Hirudinea have, like the Oligochaeta, a clitellum which, as in some of the lower members of that group, is limited to a very few segments in the immediate neighbourhood of the generative openings. It occupies inHirudosegments 9, 10, and 11. The epidermis gives rise to many unicellular glands which either remainin situor get moved to a deeper position. In this the leeches exactly resemble the earthworms. There is generally a great deal of connective tissue in the body-wall. The muscles consist of circular, longitudinal, and radial series. The individual fibres have the same structure as those of the Oligochaeta, consisting of a soft and undifferentiated core, round which is a radially-striated sheath of contractile substance.
Alimentary Canal.—The leeches are divided into the Rhynchobdellae, which have a proboscis but no jaws, and the Gnathobdellae, which possess a series of jaws but have no proboscis. But the division is not a hard and fast one, for we have Whitman's genusLeptostoma, which should belong to the jawed division, but which has quite rudimentary jaws without the sharp denticles so characteristic ofHirudo. The pharynx is furnished with salivary glands. The oesophagus is followed by the proventriculus, which has a varying number of pairs of caeca; then comes the intestine and the rectum. The anus is, as a general rule, placed dorsally to the sucker, but there are a few rare exceptions where the anus is within the sucker. The caeca are totally absent inTrocheta.
Vascular System.—As will be seen from Fig. 205, the vascular system of the Hirudinea is constructed on a plan which closely resembles that of the Oligochaeta. The diagram representsGlossiphonia, one of the Rhynchobdellae, the group which comes nearer to the Oligochaeta in many particulars than the Gnathobdellae. We can recognise a dorsal and a ventral vessel, which are united in the anterior part of the body by three perioesophageal rings, the elongation of which, particularly of the last pair (v), from before backwards is very marked. In the region of the sucker the dorsal and ventral vessels are united by fourteen shorter loops, the number of which has an interestingrelation to the number of segments out of which this portion of the body is possibly formed. It will be observed also that the dorsal vessel is double in this region, a condition which obtains along its whole length inBranchellion—a repetition of what has been described in more than one species of Oligochaete. In the region of the last pair of digestive caeca the dorsal vessel has appended to it copious sinuses which embrace the intestine and supply its walls with blood. InHirudothere are only a pair of lateral vessels, and neither dorsal nor ventral vessels; in this leech and in the Gnathobdellae generally there are intra-epidermic capillaries, a fact first discovered by Professor Lankester, and now known to occur also in the Oligochaeta.
The development of the blood-vessels shows that they have no relation whatever to the coelom, in spite of their subsequent connexion with it. The two longitudinal stems ofHirudoarise as cavities in the somatic layer of the mesoblast after the formation of the coelom. InNephelis, but not inHirudo, Dr. Bürger thinks that there is some reason for regarding the vascular system as the remains of the primitive segmentation-cavity of the embryo, an opinion which is held in respect of the vascular system of many other animals.
fig205Fig. 205.—Glossiphonia marginata, vascular and alimentary system. ×4. (After Oka.)a, Dorsal vessel;g, intestinal caecum;v, one of the hearts.
Fig. 205.—Glossiphonia marginata, vascular and alimentary system. ×4. (After Oka.)a, Dorsal vessel;g, intestinal caecum;v, one of the hearts.
Fig. 205.—Glossiphonia marginata, vascular and alimentary system. ×4. (After Oka.)a, Dorsal vessel;g, intestinal caecum;v, one of the hearts.
Body-Cavity.—One of the most marked differences between the leeches and the Oligochaeta is in the body-cavity. In the latter there are a series of cavities corresponding to the segments, which are bounded in front and behind by the intersegmental septa, and in which all the viscera lie. In leeches such an arrangement is not recognisable save inAcanthobdella, where Kowalevsky[455]has quite recently described a typical coelom divided by septa into twenty segments. In transverse sections the body of other leeches appears at first sight to be solid, owing to the growth of the muscles and connective tissue. A more careful study, however, has revealed thefact that there are considerable remains of the body-cavity or coelom which form a complicated system of spaces and channels. What has happened, in fact, in the leech is that the coelom has become gradually and partially obliterated by proliferation of the cells in the interior of the body, a process of obliteration which has already commenced in the Oligochaeta. In many of the latter, some of the principal blood-vessels have become surrounded by a space cut off from the general body-cavity, while in the majority a special cavity surrounds the testes and the funnels of the sperm-ducts. This process of the formation of separate cavities for the inclusion of the several viscera culminates in the leeches with the marked obliteration of the greater part of the coelom. This has become so much reduced to the condition of narrow tubes that there has been a tendency to confuse it with the vascular system, more especially perhaps in those forms in which the blood is tinged with haemoglobin, and in which there is a connexion between the two systems of spaces. This confusion has been further increased by the plan of injecting the vascular system, a method of investigation which must be employed with great care in delicately-organised creatures whose tissues can be easily ruptured, and so lead to a flow of the injecting fluid into places and in directions impossible during life.
fig206Fig. 206.—Coelomic canals ofGlossiphonia complanata. × 10. (After Oka.)a, Dorsal canal containing dorsal blood-vessel;b, ventral canal containing ventral blood-vessel;l, lateral canal;n, nerve-cord.
Fig. 206.—Coelomic canals ofGlossiphonia complanata. × 10. (After Oka.)a, Dorsal canal containing dorsal blood-vessel;b, ventral canal containing ventral blood-vessel;l, lateral canal;n, nerve-cord.
Fig. 206.—Coelomic canals ofGlossiphonia complanata. × 10. (After Oka.)a, Dorsal canal containing dorsal blood-vessel;b, ventral canal containing ventral blood-vessel;l, lateral canal;n, nerve-cord.
In transverse sections of leeches it may be seen in successful preparations that the various organs of the body are enclosed in spaces. The funnels of the nephridia open into lacunae which could hardly in any case be regarded as blood spaces, while the blood-vessels themselves with their muscular walls cannot be confounded, at least in the case of the larger trunks, with the spaces not having muscular walls which surround them. Furthermore, it will be pointed out immediately that the reproductive organs are produced on the walls of spaces which are the commencement in the embryo of the reduced coelom of the adult worm. These spaces therefore conform in every particular to the generalconditions which have been laid down about the characters of a true coelom. As to the complexity of this system, attention may be directed, to the accompanying diagram (Fig. 206) of the coelom of a segment ofGlossiphonia, which has been lately worked out in detail.[456]It will be observed that there are four main longitudinal sinuses which are connected by a complicated system of transverse tubes and spaces. In the anterior part of the body, before the point where the intestinal caeca arise, the dorsal and ventral lacunae fuse to form one larger so-called median lacuna. The cavity of this is interrupted, in correspondence with the segmentation of the body, by septa exactly comparable to those of Oligochaeta; but the septa inGlossiphoniaare not present at every segment. So far our account of the coelom is chiefly derived from the genusGlossiphonia. InHirudo, which is an example of the Gnathobdellae, the coelom is still further reduced; the lateral sinuses in them appear to be absent. But on the other hand there is formed a series of cavities in a form of connective tissue which has been termed botryoidal tissue. The cells of this tissue become hollowed out, and form channels which are in communication on the one hand with the remains of the coelom and on the other with the vascular system. This system has certain analogies with the lymphatic vessels of Vertebrates, and, like them, is an intermediary between the body-cavity and the blood. Originally, however, these botryoidal vessels have nothing whatever to do with either the vascular or the coelomic system; their connexion with both is a purely secondary affair, and only appears, comparatively speaking, late in life.
The development of the spaces here spoken of collectively as coelom confirms this interpretation of their nature. In the embryos ofHirudo,Aulastomum, andNephelisthere is a ventral space,[457]which includes the nerve-cord. Into this open a series of paired and segmentally-disposed lateral cavities, a pair to each segment. The ventral cavity itself is formed by fusion of the median parts of the lateral cavities. There is here clear evidence of a coelom, developed on a plan fundamentally identical with that of the Oligochaeta in that it is formed as a paired series of chambers corresponding to the segmentation of the body.
Nephridia.—The "segmental organs" or nephridia are seen intheir simplest form in such a type asGlossiphonia—the Rhynchobdellae, to which this genus belongs, being indeed in most particulars less specialised than the Gnathobdellae. Here we have a distinct funnel opening freely into the median or ventral coelomic space, which is immediately followed by a rounded swelling termed by Oka[458]the capsule; this is filled with cells, in the interstices of which the ductules are situated and meander. There is in this capsule a very strong likeness to the glandular brownish swelling which immediately follows the funnel in the nephridia of certain of the aquatic Oligochaeta, for example the Naids, where, as Vejdovsky has shown, there is a similar "rete mirabile" of the nephridial duct. After the capsule is a single row of cells which are disposed in a complicated coil. These cells are perforated by the duct, which is thus, as in the Oligochaeta, intracellular. In the first set of cells the duct is single, and gives off numerous branchlets into the interior of each cell, a condition which has also been observed in many Oligochaeta. Afterwards the cells are perforated by two, or even three, main ducts, for the duct returns upon itself and traverses the row of cells more than once; there are also branchlets developed from one or other of the main ducts. The terminal part of the nephridium is a short invagination from the exterior, which is lined by cells. There is clearly a close resemblance here with the nephridium of an Oligochaete. The nephridium, however, except for the funnel and the narrow tube immediately following it, does not appear to be ciliated.
fig207Fig. 207.—Nephridium ofHirudo medicinalis. × 10. (After Bourne.)f, Funnel;v, distal vesicle.
Fig. 207.—Nephridium ofHirudo medicinalis. × 10. (After Bourne.)f, Funnel;v, distal vesicle.
Fig. 207.—Nephridium ofHirudo medicinalis. × 10. (After Bourne.)f, Funnel;v, distal vesicle.
There is, however, some difference of opinion as to the portions of the nephridium where there are two ducts in a single cell. Bourne[459]thinks that where there are two ducts there are twocells, one lying inside the other, and that there is sometimes also a telescoping of cell within cell where the duct is single. InHirudothe same writer has described the nephridial funnel, which has lost the simple character of that ofGlossiphonia. The funnel is represented by a cabbage-head-like mass (Fig. 207,f) of ciliated cells with no single definite outlet to the exterior as inGlossiphonia. It appears to be an organ which has lost its proper function—a degeneration of the funnel being, as a matter of fact, not unknown in the Oligochaeta, where it may be carried to absolute extinction (Chaetogaster). InBranchellionandPontobdellathe simple metameric arrangement of the nephridia is to some extent lost, owing to the formation of a network continuous from segment to segment. It will be borne in mind that the Oligochaeta are the only other Chaetopods in which such a nephridial network has been stated to exist.
Male Reproductive Organs.—InHirudo medicinalisthere are nine, occasionally ten, pairs of testes, which are round white bodies arranged segmentally,i.e.a pair to each segment. From each arises a slender, somewhat sinuous tube, which enters the common collecting tube of its own side; each of these is much contorted at the upper end, the coiled portion being termed the epididymis. From this they enter a muscular penis which can be protruded. This is the arrangement met with in all leeches, save for the fact that the penis is absent in some; inGlossiphonia(see Fig. 208) this is the case. The number of pairs of testes also varies; and inNephelisthey are no longer arranged metamerically.
fig208Fig. 208.—Nervous system and reproductive organs ofGlossiphonia plana. × 2. (After Whitman.)gl, Prostate glands;n, nerve-cord;o, ovary;t, testes.
Fig. 208.—Nervous system and reproductive organs ofGlossiphonia plana. × 2. (After Whitman.)gl, Prostate glands;n, nerve-cord;o, ovary;t, testes.
Fig. 208.—Nervous system and reproductive organs ofGlossiphonia plana. × 2. (After Whitman.)gl, Prostate glands;n, nerve-cord;o, ovary;t, testes.
The testes arise as local proliferations of the epithelium of the lateral coelomic cavities, but from the somatic wall, not from the splanchnic, as in the case of the ovaries to be described later. A portion of the tissue which is to form the testis grows out laterally into a thin cord, which is to become the vas efferens of thattestis. Later both testis and duct become hollowed out with a common cavity. The main portion of the vas deferens of each side, as well as the terminal copulatory apparatus, is an ingrowth from the epidermis which meets the downgrowths from the testes.
That there are considerable differences between the reproductive organs of the leeches and those of Oligochaeta will be apparent from the above description. There are, however, to begin with, certain obvious similarities. In the first place, the origin of the reproductive glands is identical; in both groups also the efferent ducts consist of two portions—an invagination from the outside, and a formation of the proximal part of the ducts near to the glands. InMoniligaster, where—exceptionally—the testes develop on the posterior wall of their segment in close contact with the funnels of the sperm-ducts, there is no very hard and fast line to be observed between the tissues of the two. The hollowing out of the testis in the leech, and the continuity of the cavity thus formed with the duct, is a specialty of the leeches not found among the Oligochaeta.
Like many Oligochaeta, the leeches may form spermatophores in which the sperm is packed for its conveyance from one individual to another.Glossiphonia(Clepsine)plana, where the structure in question has been elaborately described by Whitman,[460]may be selected as an example. The spermatophore (Fig. 209) is about 8 mm. long, and is clearly formed of two halves, each of which is formed separately in one ductus ejaculatorius, the soldering together being effected in the common part of the male ducts, where also a basal portion with a single lumen is added. The spermatophore has a double wall. It is deposited not in the neighbourhood of the generative pores, but upon the back; and Whitman has discovered the extraordinary fact that the spermatozoa find their way through the body-wall of the leech into the interior of its body, where fertilisation presumably occurs.
fig209Fig. 209.—Spermatophore ofGlossiphonia plana. × 22. (After Whitman.)
Fig. 209.—Spermatophore ofGlossiphonia plana. × 22. (After Whitman.)
Fig. 209.—Spermatophore ofGlossiphonia plana. × 22. (After Whitman.)
Female Reproductive Organs.—The ovaries of the Hirudinea appear to differ from those of the Oligochaeta in that the ovaries are continuous with their ducts. InHirudo, however, the real ovary of each side consists of masses of germinal tissue lying freely within a sac which communicates with a duct; the two ducts unite to form a much convoluted tube which opens into a thick-walled vagina, itself opening again on to the exterior by a median unpaired opening on the seventh segment. The muscular vagina is not always present.
The median unpaired female aperture offers now no particular difficulty, since in many earthworms, e.g.Perichaeta, this orifice is in the same condition; nor does the fusion of the oviducts and the so-called ovaries; for inEudrilus, for example, and in many Eudrilidae, the ovary is contained in a sac into which the oviduct also opens. It will be noticed too that the existence of short oviducts as compared with the long sperm-ducts is a further point of likeness to at any rate the higher Oligochaeta. But a further comparison needs first to be based upon a consideration of the development of the different sections of the apparatus in the leech. The independence of the ovaries and their ducts has been proved by several observers; quite recently Bürger has dealt with the matter inNephelis,Hirudo, andAulastomum gulo.[461]He has found that the ovaries arise from the splanchnic wall of the lateral coelomic cavities; they are therefore proliferations of the coelomic epithelium, as in Oligochaeta and all Coelomates so far as is known. The peripheral layer of the mass of indifferent cells which constitutes the ovary becomes somewhat modified; its cells are flattened, and it at length separates itself and forms a capsule surrounding the other cells, which are in fact, or become, the ovary. This capsule meets and fuses with the ducts, which are invaginations from the exterior of the body.
There are clearly differences between the ovary of a leech and that of a typical Oligochaete likeLumbricus. The only point of agreement, in fact, is the origin of the reproductive gland itself from the walls of the body-cavity. InLumbricusand allied forms, whatever may be held with regard to their homologies, the oviducts as a matter of fact appear first as funnels, which afterwards bore their way to the exterior. They arepurely mesoblastic structures, not—except perhaps the very extremity—derived from an ingrowth from the epidermis. We have, however, other Oligochaeta in which there are closer resemblances to what has just been described in the Hirudinea. In more than one point the aberrant family of the Eudrilidae come nearer to the Hirudinea than any other Oligochaeta, in spite ofBranchiobdellawith its jaws and sucker. Now inEudrilusthe ovary is enclosed in a capsule which becomes continuous with the duct of the great sperm-holding pouch, itself an invagination from the exterior; there is no reason in this Annelid why the ova should not reach the exterior by this system of ducts, although there is no actual experimental proof that they do so. In any case there is also an oviduct corresponding to that ofLumbricus, which opens into the opposite side of the duct of the spermathecal pouch on the one hand, and into the receptaculum ovorum on the other; it has no direct connexion whatever with the sac containing the ovary. If we were to cut off the receptaculum and the oviduct and reduce the spermathecal sac to its duct, the result would be much the same as we find it in the leech.
Cocoon.—The practice of forming a cocoon for the shelter of the eggs and of the developing young is shared with the Oligochaeta; but not all leeches deposit their eggs in this manner.Glossiphonia, for instance, carries its eggs upon the ventral face of the body, where the young remain for some time after they are hatched attached by the posterior sucker to their parent's body, and from which situation of safety they make short excursions. Other leeches deposit their eggs singly, but agglutinated together upon stones, etc. In the medicinal leech the cocoon is ovoid in shape, and from the end, which is closed by a temporary plug, the young when ready escape. This cocoon is deposited, as is that of an earthworm, in soil near to the borders of a marsh or pond, so that the young, while enjoying the requisite degree of moisture, may not be injured by a too wet environment. On the other handPontobdellaand some other leeches lay their cocoons attached to bodies actually submerged in the water. The cocoon is secreted, as in Oligochaeta, by the clitellum, and as in them, is drawn off over the head, the ova and sperm probably flowing into it during the process. The elasticity of the slightly hardened mucus causes the two ends of the cocoon toclose up when it is free from the body; it is then whitish and soft, and the leech fixes it, and appears to polish the surface with its buccal sucker. In a few hours the cocoon becomes amber brown, a colour which characterises the cocoons of a great many earthworms and other Oligochaeta. The medicinal leech forms its cocoon in the same way as doesNephelis, to which the above description refers; but when the cocoon is formed the leech covers it with another layer of mucus, which Vaillant, from whose work the foregoing notes are extracted,[462]thinks may be produced from the so-called salivary glands.
Classification.—The number of different kinds of leeches is at present uncertain. Seeing that no less than sixty-four varieties of the commonHirudo medicinalis—colour varieties, it is true—are said to exist, it is not wonderful that the labours of some systematists have been severe, and have provoked much criticism and alteration on the part of others.[463]As to genera, Vaillant, in his recent continuation of de Quatrefages' "Annelés" in theSuites à Buffon, which includes the literature up to the year 1886, allows thirty-seven, some (three) of which, however, are admitted to beincertae sedis. Blanchard, who has paid a great deal of attention to the group, reduces these by six, which he considers to be synonyms; but on the other hand he has added or rescued from oblivion six or seven others, and Whitman has instituted several Japanese and Australian genera. Most of these generic types are, however, only imperfectly known, and from external characters only. It is quite problematical how many valid genera should be retained; in the meantime those that are fairly well known are divided by Blanchard[464]in the following way:—
Sub-Order 1. Rhynchobdellae.—Hirudinea with an exsertile proboscis, without jaws, and with colourless blood.
Fam. 1.Ichthyobdellidae.—Body formed of two regions, a narrower anterior portion and a wider "abdomen," both anterior and posterior suckers distinct from the body.
These leeches are parasites of fishes and of some other animals such as tortoises. The family contains a number of genera.Branchellion(Fig. 204) has a series of leaf-like branchiae on both sides; inCystibranchusthe respiratory organs are reduced to a series of round vesicles. The latter genus occurs in Europe and North America, and is parasitic upon marine and fresh-water fishes.Piscicolais a common leech which confines its attacks to the inhabitants of fresh water.Pontobdella(Fig. 210) is marine, and affects rays and sharks; the best known species,P. muricata, is usually of a green colour.
To the family Ichthyobdellidae also belongs the large Chilian leechMacrobdella valdiviana, of which there is or are also species in North America. Philippi's figure of this leech[465]shows the distinct neck; and as it has no jaws, it should be referred to the present family. It has got an undue reputation for size, 2½ feet[466]having been assigned to it. As a matter of fact Philippi's illustration depicts an Annelid of about 7 inches in length, with a greatest diameter of about an inch. But doubtless when extended in walking it would be longer—1½ feet, Philippi thinks. It has no eyes, a failing which is not unusual among the leeches.
Fam. 2.Glossiphoniidae.—Anterior sucker fused with the body, posterior sucker distinct. No cocoon.