Page:Encyclopædia Britannica, Ninth Edition, v. 21.djvu/16

Rh 6 ROTIFEKA present the flaps found on the tail of other forms. Pedalion also has a small ciliated muscular process (fig. 3, A, g) placed immediately below the mouth, and termed a " chin," which appears to be merely a greater development of a sort of lower lip which occurs in many Rotifers. Muscular System. All the Eotifera present a muscular system which is generally very well developed. Transverse striation occurs among the fibres to a varying extent, being well marked in cases where the muscle is much used. The muscles which move the body as a whole are arranged as circular and longitudinal series, but they are arranged in social groups and do not form a com- plete layer of the body-wall as in the various worms. Some of the longitudinal muscles are specially developed in connexion with the tail or pedicle. Other muscles are developed in connexion with special systems of organs, the trochal disks, the jaw apparatus, and the reproductive system. The muscles in connexion with the trochal disK serve to protrude or withdraw it, and to move it about, when extruded, in various directions. The protrusion is probably, however, generally effected by the elasticity of the integument coming into play during the relaxation of the retractor muscles, and by a general contraction of the body wall. The tentaculiferous apparatus of Polyzoa and Gephyrea is protruded in the same manner. Trochal Disk. This structure is the peculiar characteristic of the class. It is homologous with the ciliated bands of the larvce of Echinoderms, Chsetopods, Molluscs, &c., and with the tenta- culiferous apparatus of Polyzoa and Gephyrea, and has been termed in common with these a " velum. " This velum presents itself in various stages of complexity. It is found as a single circum-oral ring (pilidium), as a single prse-oral ring (Chsetopod larvae), or as a single prse-oral ring coexisting with one or more post-oral rings (Chsetopod larvte, Holothurian larvae). We may here assume that the ancestral condition was a single circum-oral ring associated with a terminal mouth and the absence of an anus, and that the exist- ence of other rings posterior to this is an expression of metameric segmentation, i.e., a repetition of similar parts. With the develop- ment of a prostomiate condition a certain change necessarily takes place in the position of this band: a portion of it comes to lie longitudinally; but it may still remain a single band, as in the larva of many Echinoderms. How have the other above-mentioned conditions of the velum come about ? How has the prae-oral band been developed ? Two views have been held with regard to this question. According to the one view, the fact whether the single band is a pra-oral or a post-oral one depends upon the position in which the anus is about to develop. If the anus develops in such a position that mouth and anus lie on one and the same side of the band, the latter becomes prse-oral ; if, however, the anus develops so that the mouth and anus lie upon opposite sides of the band, the band becomes post-oral. If we hold this view we must consider any second band, whether pre- or post-oral, to arise as a new development. The other view premises that the anus always forms so as to leave the primitive ring or "architroch" post-oral, i.e., between mouth and anus. Concurrently with the development of a prostomium this architroch somewhat changes its position and the two lateral portions come to lie longitudinally ; these may be supposed to have met in the median dorsal line and to have coalesced BO as to leave two rings the one pra-oral (a " cephalo- troch"), the other post-oral (a " branchiotroch ") ; this latter may atrophy, leaving the single prae-oral ring, or it may become further developed and thrown into more or less elaborate folds. The exist- ing condition of the trochal disk or velum in the JRotifera seems to the writer of this article to bear out the latter view as to the way in which modifications of the velum tfay have come about. In its simplest condition it forms a single circum-oral ring, as in Microcodon (fig. 1, D). The structures at the sides of the mouth in this form are stated to be bristles, and have therefore nothing to do with the velum (fig. 4, A, p). This simple ring may become thrown into folds, so forming a series of processes standing up around the mouth ; this is the condition in Stephanoceros (fig. 4, B, p). There are, however, but few forms presenting this simple condi- tion ; and it must be remembered that the evidence for the assump- tion here made, that this is a persistent architroch and not a bran- chiotroch persisting where a cephalotroch has vanished, is not at present conclusive. This band, may, while remaining single and perfectly continuous, become prolonged around a lobe overhanging the mouth a prostomium. This condition occurs in Philodina (fig. 4, E, F, p); the two sides of the post-oral ring do not meet dorsally, but are carried up and are continuous with the row of cilia lining the " wheels. " There is thus one continuous ciliated band, a portion of which runs up in front of the mouth. This condition corresponds to that of the Auricularian larva. The fold- ing of the band has become already somewhat complicated ; f a hypothetical intermediate condition is shown in fig. 4, c, D. The next stage in the advancing complexity is that the prostomial por- tion of the band (fig. 4, o, H, p) becomes separated as a distinct ring, a cephalotroch ; we find such a stage in Lacinularia (fig. 4, G, H), where both cephalotroch and branchiotroch remain fairly simple in shape. In Melicerta (fig. 4, i, j) both cephalotroch and brauchiotroch are thrown into folds. Lastly, we find that in such forms as Brachionus the cephalotroch becomes lirst convoluted and in v c FIG. 4. Diagrams of the Trochal Disk. A, Microcodon. B, Stephanoceros ; the mouth lies in the centre of a group of tentacles. C, hypothetical intermediate form between ificrocodon and Philodina, showing the development of a pro- stomial portion of the velum. D, dorsal view of the same. E, Philodina. F, dorsal view of the same. G, Lacinularia the dotted line represents the por- tion of the velum which has become separated as a special ring a cephalotroch. H, dorsal view of the same. I, Melicerta ; the doited lino represents the cephalotroch; both this and the branchiotroch have become thrown into folds. J, dorsal view of the same. K,* 'Brachionus ; there is a large prie-oral lobe with three ciliated regions, shown by the dotted lines c, c, a discontinuous cephalotroch. L, dorsal view of the same. m, mouth ; p, p', velum ; p, architroch ; p', portion of the architroch which becomes carried forward to line the prostomial region, but does not become separated ; c, cephalotroch. (Original.) then discontinuous (fig. 4, K, L, c), and further it may become so reduced as to be represented only by a few isolated tufts, as in Asplanchna (fig. 1, i, x and a;'); in such a form as Lindia (fig. 6, c) the branchiotroch has vanished and the cephalotroch has become reduced to the two small patches at the sides of the head. The trochal apparatus serves the Rotifcra as a locomotive organ and to bring the food particles to the mouth ; the cilia work so as to produce currents towards the mouth. Digestive System. This consists of the following regions: (1) the oral cavity ; (2) the pharynx ; (3) the oesophagus ; (4) the stomach ; (5) the intestine, which terminates in an anus. The anus is absent in one group. The pharynx contains the mastax with its teeth ; these are calcareous structures, and are known as the trophi. In a typical mastax (8, 9) (Bra- i chionus, fig. 5, A) -f there are a median anvil or incus and two hammer-like portions, mallei. The incus consists of two rami (e) resting upon a cen- tral fulcrum (f) ; each malleus con- sists of a handle or manubrium (c) and a head or uncus (d), which often presents a comb- like structure. Fig. 5 shows some of ., FIG. 5. Trophi of various forms : A, Brachionus : B, modlhcatiouswhich D iglena forcipata ; C, Asplanchna ; D, Philodina. f, the apparatus may fulcrum, and e,e, rami, forming the incus; c, manubrium, exhibit. The parts an ^ d > un cus, forming the malleus. (After Hudson.) may become very slender, as in Diglena forcipata (fig. 5, B) ; the mallei may bo absent, as in Asplanchna (fig. 5, c), the rami being highly developed into curved forceps and movable one on the other ; or, the manubria being absent and the fulcrum rudimentary, the rami may become massive and subquadratic, as in Philodina (fig. 5, D). All the true Rotifers possess a mastax. Ehrenberg's group of the Agomphia consisted of a heterogeneous collection of forms, Icltthydium and Chsetonotus being Qastrotricha, and Cyphonautes