Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/722

Rh 680 W K M foration, being absent. The larval excretory organs of the Hirudinca, like these of the Oligochxta referred to above, are to be looked upon as in a more rudimentary condition ; they are unbranched, and arc sometimes without an external orifice. Moreover, essentially similar organs are found in the larval mollusc. In many of the above- cited examples it is certain that the larval excretory organs have no connexion with the permanent excretory organs ; they atrophy before the latter appear. Hatchek, however, has stated that there is a connexion between the larval and permanent excretory organs in Polygordius; doubts have been thrown upon this observation by some who believe that the facts already cited showed (1) that the excretory system of Annelida and Hirudinca is a new formation, while (2) the excretory system of their Platyhelminth ancestors is represented by the transitory excretory system of the Annelida, which has therefore naturally no connexion with the permanent excretory system. This view has the merit of explaining the presence of apparently similar structures (i.e., the larval nephridia) in such diverse types as Mollusca, Hirudinca, and Gcpliyrca, and is perhaps further supported by the high development of the larval excretory organ in the active larv;e of Polygordius, Echiurus, and the Ch&topoda, and its rudimentary character in the embryo Oligochxta. It follows from this that the permanent nephridia of the CliB&topoda are new structures, unless the views of Bergh (18) be accepted, who would derive these organs from the generative ducts of the Platyhelminths. Against this hypothesis may be urged (1) the unlikelihood of a new formation of excretory organs in Annelids, and the probability of these organs being really homologous with those of their Platy helminth ancestors, and (2) the fact that the larval excretory organs of Polygordius, Polychssta, and Oligochseta and Hirudinca are con nected with the permanent system or at least are not in any way replaced by the permanent excretory system ; in the Oligoch&ta the larval excretory organs appear comparatively late, and the segment occupied by them never gives rise to a pair of permanent nephridia. The following facts lead to another hypothesis, which is in many respects more acceptable. A connexion between the nephridia of consecutive segments has been recently stated by AVilson to occur in the embryo Lumbricus. Meyer and Cunningham have observed the same in Tcrcbclla. Vejdvosky (15) has recorded in Anachxta bokemica a connexion between the nephridia of the 21st and 22d segments, and moreover the first pair of nephridia has two internal funnels. In the leech Pantobdella the nephridia (Bourne, Q. J. Min. Sci., 1884) form a network, the internal funnels and external apertures alone being arranged metamerically. The presence of numerous external pores to each segment in certain earthworms, and the continuity of nephridia of adjacent segments are facts to be referred to the same category. Eisig s discovery of the presence of many nephridia in each of the seg ments of the Capitellidce, which are connected together, is also, like the other facts referred to, in harmony with the supposition that the excretory system of Ilia Annelida has been directly derived from that of the Platyhelminths somewhat as follows. The excretory system is at first, as in the Platyhelminth, a continuous system, opening by numerous apertures into the body cavity by a single orifice or a pair of orifices on to the exterior. Secondary external apertures are then formed, which are irregular in their disposition (these actually occur in certain Platyhelminths), and more or less numerous ; this condition is largely retained in Acantliodrilus and Pcrichxta; the secondary external apertures as well as the internal apertures then become reduced in number and metamerically arranged ; this condition occurs in Pontobdclla and Terebclla, and to a very limited extent in Anack&ta. The connexion between the nephridial system of succession segments then disappears, and the characteristic Annelid excretory system is arrived at. Alimen- Alimentary Tract. The alimentary canal of all the Oligochtcta is a straight tube running from mouth to anus, but even in the lowest forms is specialized into different regions. A pharynx, oesophagus, and intestine can be recognized universally ; the pharynx is formed by the stomodseal invagination of the epiblast, while the terminal section of the intestine is formed by the proc- todfeal invagination; the rest of the alimentary canal is hypo- blastic. In JEolosoma the pharynx is restricted to the first segment of the body, a condition which is seen in the embryonic stages of other OligochfKta, but also in the adult Polygordius (see below). This is followed by the narrow oesophagus, which leads, in the fourth segment, into the intestine ; the intestine is at first wide. but afterwards becomes narrower; the whole alimentary canal is ciliated. In the higher types the pharynx occupies several sef- ments, and is preceded by a buccal cavity, the epithelium of which s not ciliated ; in many OligochiKta(e.g., Enchytrseus) the pharynx is protrusible. It is frequently furnished with glands, which are of two kinds, and probably not morphologically comparable. In many Lnchytrssidae. and Naidomorpha certain of the anterior segments contain glands attached to the anterior faces of the interse^mental septa ; these have been termed septal glands. In Anachseta there are only two pairs of these glands, but four in Pachydrilus ; the glands of each side of the body are connected by a continuous longitudinal duct which opens into the pharynx. Similar glands appear to occur in most Lumbritidx in the form of unicellular glands attached to the pharynx. Another series of glandular structures are connected with the pharynx, which have been termed by Vejdovsky salivary glands ; these are found in the Enchytrieidx, and consist of simple or branched tubes, which open into the hinder end of the pharynx at each side by a single duct ; since these glands agree in their minute structure with nephridia, which are not found in the segments which contain the glands, it is probable that they represent slightly metamorphosed nephridia (Vejdovsky). Among earthworms Uro- chs&ta, Diachs&ta, and Acantliodrilus multiporus possess a pair of glands at the anterior end of the body (glandes a mueosite, Perrier), which are larger and more complicated than the nephridia, though their structure is the same ; in Acantliodrilus these glands open into the buccal cavity; in Urochiuta they are branched and open on to the surface of the body on the one hand, and into the coelom by several funnels (Bcddard). It is possible that they are the homo- logues of the salivary glands in the Enchytrs&idx. The oesophagus is ciliated in the lower forms, but among earth worms cilia appear to be limited to that section of the oesophagus which lies behind the gizzard. The intestine, however, appears to be ciliated in all the Oligochxta. There is thus a gradual diminu tion in the ciliation of the alimentary tract in passing from the lower to the higher forms. In jEolosoma the whole canal from mouth to anus is ciliated. When the buccal cavity first appears it is lined with a cuticle, and has no cilia. The pharynx loses its cilia in the Enchytraeidse, while the rest of the alimentary tract is cili ated. Among earthworms the cilia are partially wanting in the oesophagus, while the intestine is lined with a ciliated epithelium. In some Oligochseta the oesophagus is furnished with a muscular dilatation, which is usually termed gizzard. This organ first makes its appearance among the Naidomorpha, but is absent from other genera of &quot; Limicolx.&quot; It is, on the contrary, present in nearly all earthworms ; the gizzard is lined with a tall columnar epithelium, which secretes a specially thickened cuticle, and the muscular layers are enormously increased. In Lumbricus the gizzard lies at the posterior end of the oesophagus, but in all other earthworms it is. succeeded as well as preceded by a section of oesophagus, as in Nais. In Lumbricus, as well as in many other genera of earth worms, the gizzard occupies two segments, and the septum dividing these segments has disappeared, or is at most represented by a few bands of muscle, which bind down the gizzard to the body- wall. On the other hand, in some earthworms as well as in the Naidomorpha the gizzard only occupies one segment. In Digaster and Trigastcr there are, as the names of these genera imply, two or three separate gizzards, while in Moniligctster there are four or five. In Trigastcr and Moniligastcr each gizzard only occupies a single segment; it is possible that the single gizzard of Lumbricus, &c., which occupies two segments, is due to the fusion of two separate gizzards lying in as many consecutive segments. The oesophagus in most earthworms is furnished with from one to six pairs of glandular diverticula, which are known as &quot;calci- ferous glands &quot; or &quot;glands of Morren.&quot; Those glands produce a calcareous secretion. 1 The intestine is wider than the oesophagus, but has much the same structure ; in Alolosoma the walls of the intestine consist of little more than a single layer of ciliated cells, but in the higher forms this is surrounded by a layer of circular and longitudinal muscular fibres. A remarkable peculiarity distinguishes the intestine of the majority of earthworms ; this is a longitudinal fold on the dorsal side which projects into the lumen of the intestine, and which is known as the &quot; typhlosolo&quot; (fig. 3, t). A typhlosole does not exist in Pontodrilus nor in any known limi- colous form. In Pcrich&ta the intestine is furnished with one or more pairs of short creca ; in Megascolex and other genera there are series of compact glands opening into the intestine. Reproductive Organs. The Oligochxta are, .so far as is known, Repro- invariably hermaphrodite ; in this particular they diifer from the ductivs Polychscta, where as a rule the testcs and ovaries are found in organs, distinct individuals. Among the Polych&ta, however, Protula and other Scrpulidse are hermaphrodite. The reproductive organs consist of testes and ovaries, with their ducts, and spermatheca?, which are filled with spermatozoa during copulation. The repro ductive glands are developed as a proliferation of the peritoneal epithelium, but are restricted to one or two segments ; these organs moreover have a definite form, and are commonly surrounded by a layer of cells of different nature from the sexual cells which make up the substance of the gland. (1) Male Reproductive, Organs (a) Testcs. In Lumbricus these organs consist of two minute pairs of solid cellular masses attached close to the median ventral line upon the posterior face of the septa which divide segments 9-10 and 10-11 (fig. 4). These organs were first discovered by Hcring. In the majority of earthworms (Acantho- 1 For an account of the function of these glands, as well as of the part which earthworms play as geological agents, see Darwin, Formation of Vegetable Mould, &c.