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PHYLUM TROCHELMINTHES 



319 



FIG. 270. Pharynx of Brachionus 

 rubens. /. fulcrum ; m. manubrium ; 

 u. uncus ; r. ramus. (After Hudson and 

 Gosse.) 



There is no continuous muscular layer, but several bands of 

 unstriped muscle (m.) pass from the lorica to the trochal disc 

 in front and to the tail behind, and act as retractors of those organs. 

 Digestive Organs. The mouth (Fig. 272, mth.) lies, as 

 already mentioned, in the ventral region of the trochal disc, anterior 

 to the ciliary circlet but posterior to the three ciliated lobes ; it leads 



by a short buccal cavity into a 

 pharynx (ph.) of peculiar struc- 

 ture known as the mastax, and 

 constituting one of the most 

 characteristic organs of the class. 

 The mastax is a muscular 

 chamber (Fig. 270) of rounded 

 form, and contains, as a thicken- 

 ing of its cuticular lining, an 

 elaborate apparatus for tri- 

 turating the food. In the 

 middle line is a forked struc- 

 ture, the incus, consisting of a 

 small base or fulcrum (j.) and of two branches or rami (r.). On 

 either side of the incus is a hammer-like structure, the malleus, 

 consisting of a handle or manubrium (m.) and of a toothed head or 

 uncus (u.). By means of the muscular walls of the chamber the 

 heads of the mallei are worked backwards and forwards upon the 

 forked uncus, and thus reduce the organ- 

 isms taken as food to a fine state of 

 division. 



The pharynx leads by a short gullet into a 

 spacious stomach (Fig. 272, sZ.) having a wall 

 composed of very 

 large epithelial 

 cells, ciliated inter- 

 nally : with it are 

 connected paired 

 digestive glands. 

 The stomach opens 

 into a rounded in- 

 testine (int.), also 

 ciliated internally, 

 which communi- 

 cates, by means of 

 a short cloaca (cl.) 

 with the ex- 

 terior. The stomach 

 and intestine are formed from the archenteron of the embryo and 

 are therefore lined by endoderm : the rest of the enteric epithelium 

 is ectodermal, the pharynx being derived from the stomodseum, 



cy, 



FIG. 271. Brachionus rubens. A, male ; B, female, with 

 attached eggs. c. gl. cement-glands ; cv. contractile vesicle ; 

 nph. nephridial-tube ; ov. ovum in body ; ov*. ova attached to 

 base of tail ; p. penis ; t. tail ; is. testis. (After Hudson and 

 Gosse.) 



