ROTIFERA 



91 



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 

 I 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 loiigitudinal series, 

 but they, are arranged in special 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 larvse 

 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 larvse), or as 

 a single prae-oral ring coexisting with one or more post-oral rings 

 (Chsetopod larvae, 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 abotit ? How has the prse-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 praj-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 prae-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 pr»- 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 so as to leave two rings — the one prae-oral (a " cephalo- 

 troch"), the other post-oral (a " branchiotroch ") ; this latter may 

 atrophyj 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 Sotifera seems to 

 the writer of this article to bear out the latter view as to the way 

 in which modifications of the velum may 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,^). 

 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 hot a bran- 

 chiotroch persisting where a cephalotrooh 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, B, 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 ; 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, g, h, p') becomes separated as a distinct 

 ring, a cephalotrooh ; we find such a stage in Lacinularia (fig. 4, 



G, h), where both cephalotrooh and branchiotroch remain fairly 

 simple in shape. In Melicerta (fig. 4, i, j) both cephalotrooh and 

 branchiotroch are throAvn into folds. Lastly, we find that in such 

 forms as Brachionus the cephalotrooh becomes first convoluted and 



■ni- 



FiG. 4. — ^Diagrams of the Trochal Disk. A, Microcodon, B, St^hanoceros ; the 

 mouth lies in the 'centre of a group of tentacles. C, hypothetical intermediate 

 form hetween Microcodon 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 tlie por- 

 tion of the velum which has become separated as a special ring — a cephalotrooh. 

 H, dorsal view of the same. I, Melicerta ; the dotted line represents the 

 cephalotrooh; both this and the branchiotroch have become thrown into folds. 

 J, dorsal view of the same. K,^Brachionus ; there is a large prse-oral lobe 

 with three ciliated regions, shown by the dotted lines c, c, a discontinuous 

 cephalotrooh. L, dorsal view of the same. 



m, mouth ; p, p\ velum ; j?, architroch ; p', portion of the architroch which 

 becomes carried forward to line the prostomial region, but does not becpme 

 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 id); in such a form as Idndia (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 Eotifera 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 cesophagns ; (4) the 

 stomach ; (5) the intestine, which terminates in an anus. The 

 anus is absent in one group. 



The pharynx contains the irtMStax with its teeth ; these are 

 calcareous structures, and are known as the trophi. In a typical 

 mastax (8, 9) (Bra- 

 chionus, fig. 5, A) 

 there are a median 

 anvil or inmus and 

 two hammer-like 

 portions, mallei.' 

 The incus consists 

 of two rami (e) 

 resting upon a cen- 

 tral fulcrum (/) ; 

 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 _ _ 



the most important p,^ 5._Trophi of 



various forms : A, Brachionus ; 



i.n I. ' T . - ni*. y. — irupiii oi variuua lorins : A, DjaciiKmiis i B, 



moomcationswmcn Digiena forcipala ; 0, Asplanchna ; D, Philodina. /, 

 the apparatus may fulcrum, ande,e,rami,formingtheincus:c,manubrium, 

 exhibit. The parts *°^ ^j uncus, forming the malleus. (After Hudson.) 

 may become very slender, as in Digiena forcipata (fig. 5, b) ; the 

 mallei may be 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. Ehreuberg's group 

 of the Agomphia consisted of a heterogeneous collection of forms, 

 — Ichthydium and Chsetonotus being Gastrotricha, and Oyphonawtes 



