vii PHYLUM TROCHELMESTHES 325 



size. The trochal disc is not perfectly symmetrical, but has at one 

 part of its circumference a depression in which the mouth lies : 

 this marks the ventral surface. The anus (a.) is dorsal in position, 

 and is placed at the junction of the tail with the trunk. 



The body-wall consists of an epidermal layer, without cell- 

 limits, covered by a chitinoid cuticle : it is by a thickening of the 

 latter in the region of the trunk that the lorica is produced. 

 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. 267, wA.) 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 structure 

 known as the mastax, and constituting one of the most character- 

 istic organs of the class. The mastax is a muscular chamber 

 (Fig. 265) of rounded form, 

 and contains, as a thickening 

 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 (/.) and 

 of two branches or rami (r.). 

 On either side of the incus is 

 a hammer-like structure, the 



j, ... PI FIG. 265 Pharynx of Brachionus rubens. 



ma/leUS, Consisting OI a handle /. fulcrum; m. manubrium; . uncus; r. 



or manubrium (m.) and of a 



toothed head or iincus (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 organisms taken as food to a fine state of 



division. 



The pharynx leads by a short gullet into a spacious stomach (st.), 

 having a wall composed of very large epithelial cells, ciliated 

 internally : with it are connected paired digestive glands. The 

 stomach opens into a rounded intestine (int.), also ciliated internally, 

 which communicates, 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, the cloaca from the proctodasum. Between the 

 body- wall and the enteric canal is a spacious body-cavity contain- 

 ing a fluid which serves the purpose of blood and contains minute 

 granules. 



