Page:EB1911 - Volume 05.djvu/835

 network places these sinuses and the lateral sinuses in communication. Here also the blood system has no communication with the sinus system of the coelom. In Hirudo and the Gnathobdellidae there is only one system of cavities which consist of four principal longitudinal trunks, of which the two lateral are contractile, which communicate with a network ramifying everywhere, even among the cells of the epidermis. The network is partly formed out of pigmented cells which are excavated and join to form tubes, the so-called botryoidal tissue, not found among the Rhynchobdellidae at all. It seems clear from the recent investigations of A. G. Bourne and E. S. Goodrich that the vascular system and the coelom are in communication (as in vertebrates by means of the lymph system). On the other hand, it has been held that in these leeches there is no vascular system at all and that the entire system of spaces is coelom. In favour of regarding the vascular system as totally absent, is the fact that the median coelomic channels contain no dorsal and ventral vessel. In favour of seeing in the lateral trunks and their branches a vascular system, is the contractility of the former, and the fact of the intrusion of the latter into the epidermis, matched among the Oligochaeta, where undoubted blood capillaries perforate the epidermis. A further fact must be considered in deciding this question, which is the discovery of ramifying coelomic tubes, approaching close to, but not entering, the epidermis in the Polychaete Arenicola. These tubes are lined by flattened epithelium and often contain blood capillaries; they communicate with the coelom and are to be regarded as prolongation of it into the thickness of the body wall.

Gonads and Gonad Ducts.—The gonads and their ducts in the Hirudinea invariably form a closed system of cavities entirely shut off from the coelom in which they lie. There is thus a broad resemblance to the Eudrilidae, to which group of Oligochaeta the Hirudinea are further akin by reason of the invariably unpaired condition of the generative apertures, and the existence of a copulatory apparatus (both of which characters, however, are present occasionally in other Oligochaeta).

The testes are more numerous than the ovaries, of which latter there are never more than one pair. The testes vary in numbers of pairs. Four (Ozobranchus) to six (Glossiphonia) or ten (Philaemon) are common numbers. In Acanthobdella, however, the testes of each side of the body have grown together to form a continuous band, which extends in front of external pore. Each testis communicates by means of an efferent duct with a common collecting duct of its side of the body, which opens on to the exterior by means of a protrusible penis, and to which is sometimes appended a seminal vesicle. The efferent ducts are ciliated, and there is a patch of cilia at the point where they communicate with the cavity of each testis. The ovaries are more extensive in some forms (e.g. Ozobranchus) than in others, where they are small rounded bodies. The two ducts continuous with the gonads open by a common vagina on to the exterior behind the male pores. This “vagina” is sometimes of exaggerated size. Thus, in Philaemon pungens (Lambert) it has the form of a large sac, into which open by a single orifice the conjoined oviducts. From this vagina arises a narrow duct leading to the exterior. In Ozobranchus the structures in question are still more complicated. The two long ovarian sacs communicate with each other by a transverse bridge before uniting to form the terminal canal. Into each ovarian sac behind the transverse junction opens a slender tube, which is greatly coiled, and, in its turn, opens into a spherical “spermathecal sac.” From this an equally slender tube proceeds, which joins its fellow of the opposite side, and the two form a thick, walled tube, which opens on to the exterior within the bursa copulatrix through which the penis protrudes. These two last-mentioned types show features which can be, as it seems, matched in the Eudrilidae.

The gonads develop (O. Bürger) in coelomic spaces close to nephridial funnels, which have, however, no relation to the gonad ducts. The ovaries are solid bodies, of which the outer layer becomes separated from the plug of cells lying within; thus a cavity is formed which is clearly coelom. This cavity and its walls becomes prolonged to form the oviducts. A stage exactly comparable to the stage in the leeches, where the ovary is surrounded by a closed sac, has been observed in Eudrilus. In this Annelid later the sac in question joins its fellow, passing beneath the nerve cord exactly as in the leech, and also grows out to reach the exterior. The sole difference is therefore that in Eudrilus the ovarian sac gives rise to a tube which bifurcates, one branch meeting a corresponding branch of the other ovary of the pair, while the second branch reaches the exterior. In the leech the two branches are fused into one. We have here clearly a case of a true coelomoduct performing the function of an oviduct in both leeches and Eudrilidae. The facts just referred to suggest further comparisons between the Hirudinea and Eudrilidae. The large sacs which have been termed vagina are suggestive of the large coelomic spermathecae in Eudrilids, a comparison which needs, however, embryological data, not at present forthcoming, for its justification. It is at least clear that in Ozobranchus this comparison is justifiable; but only probable, or perhaps possible, in the case of Philaemon. In the former, the duct, leading from the ovarian sac, and swelling along its course into the spherical sac, the “spermatheca,” is highly suggestive of the oviduct and receptaculum of the Eudrilidae.

The testes during development become hollowed out and are prolonged into the vasa efferentia. These ducts therefore have not their exact counterparts in the Oligochaeta, unless we are to assume that they collectively are represented by the seminal vesicles of earthworms and the vasa deferentia. It is to be noted that the Hirudinea differ from the Oligochaeta in that the male pore is in advance of the gonads (except in Acanthobdella, which here, as in so many points, approximates to the Oligochaeta), whereas in Oligochaeta that pore is behind the gonads (again with an exception, Allurus).

Classification.—The Hirudinea may be divided into three families:—

(i.) Rhynchobdellidae.—A protrusible proboscis exists, but there are no jaws. The blood is colourless. Pontobdella, Glossiphonia, &c.

(ii.) Gnathobdellidae.—A proboscis absent, but jaws usually present. Blood coloured red with haemoglobin. Hirudo, Nephelis, &c.

(iii.) Acanthobdellidae.—Proboscis present, but short. Paired setae of Oligochaetous pattern present in anterior segments. Blood red. Acanthobdella.

.—A. O. Kovalevsky, ''Bull. Imp. Sci. (St Petersburg, November 1896) (Acanthobdella); A. G. Bourne, Quart. Journ. Micr. Sci., 1884; A. Oka, Zeitschr. wiss. Zool., 1894; E. S. Goodrich, Quart. Journ. Micr. Sci., 1899; W. E. Castle, Bull. Mus. Comp. Zool., 1900; A. M. Lambert, Proc. Roy. Soc. (Victoria, 1897); C. O. Whitman, Journ. Morph., 1889 and 1891; O. Bürger, Zeitschr. wiss. Zool.'', 1902, and other memoirs by the above, and by St V. Apáthy, R. Blanchard, H. Bolsius, A. Dendy, R. S. Bergh, &c.

 

CHAETOSOMATIDA, a small group of minute, free-living, aquatic organisms which are usually placed as an annex to the Nematoda. Indeed Mechnikov, to whom we owe much of our knowledge of these forms, calls them “creeping Nematoda.” They are usually found amongst seaweed in temperate seas, but they are probably widely distributed; some are fresh-water. The genus Chaetosoma, with the two species ''Ch. claparedii and Ch. ophicephalum and the genus Tristicochaeta'', have swollen heads. The third genus Rhabdogaster has no such distinct head, though the body may be swollen anteriorly. The mouth is terminal and anterior and surrounded by a ring of spicules or a half-ring of hooks. Scattered hairs cover the body. Just in front of the anus there is in Chaetosoma a double, and in Tristicochaeta a triple row of about fifteen stout cylindrical projections upon which the animals creep. The females are a little larger than the males; in ''Ch. claparedii'' the former attain a length of 1.5 mm., the latter of 1.12 mm. The mouth opens into an oesophagus which passes into an intestine; this opens by a ventral anus situated a little in front of the posterior end. The testis is single, and its duct opens with the anus, and is provided with a couple of spicules. The ovary is double, and the oviducts open by a median ventral pore about the middle of the body; in this region there is a second swelling both in Chaetosoma and in Rhabdogaster. The last-named form is in the female 0.36 mm. in length. In it the hairs are confined to the dorsal middle line and the creeping setae are hooked, of a finer structure than in Chaetosoma, and situated so far forward that the vagina opens amongst them. ''Ch. ophicephalum'' has been taken in the English Channel.

 CHAFER, a word used in modern speech to distinguish the beetles of the family Scarabaeidae, and more especially those species which feed on leaves in the adult state. The word is derived from the O. Eng. ceafor, and it is interesting to note that the cognate Ger. Käfer is applied to beetles of all kinds. For the characters of the Scarabaeidae see. This family includes a large number of beetles, some of which feed on