PHYLUM TROCHELMINTHES 



333 



close. Lastly, both the pre- and post-oral circlets may be pro- 

 duced into more or less complex lobes, as in Melicerta (Fig. 269, Ii), 

 or may be interrupted as in Brachionus, in which the pre-oral 

 circlet is represented by three distinct lobes, or as in Pedalion, 

 in which both circlets are divided into right and left moieties. 

 In one genus the trochal disc is absent. 



Digestive Organs. — The typical form of mastax or pharyngeal 

 mill is that described in Brachionus (Fig. 265). There is an un- 

 paired incus consisting of a short stem or fulcrum (/) and of two 

 broad branches or rami (r), and a pair of mallei, each consisting 

 of a stout handle or manubrium (m) and a broad, toothed head or 

 tmcus (u). In some forms all the parts of the apparatus become 

 very slender, the incus assuming the form of forceps (Fig. 27i, A). 

 Or the mallei may be absent and the two rami movable upon 

 one another so as to convert the incus into a pair of forceps (B) 



Fig. 271. — Typical forms of mastax. 

 /. fulcrum ; m. manubrium ; 



A, forcipate type ; B, incudate type ; C, ramate type, 

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



used to seize prey, the mastax being in this case protrusible. 

 Lastly, the fulcrum and manubrium may be absent, and the unci 

 and rami very strong and massive (C). Glands, supposed to be 

 salivary, open into the mastax or oesophagus. 



The stomach is always large, and usually has a pair of digestive 

 glands opening into it : it may pass insensibly into the intestine, 

 or the latter may be a distinct chamber of more or less globular 

 form. In the Rhizota the intestine turns forwards so as to allow of 

 the anus being brought over the edge of the tube in defaecation 

 (Fig. 269, 4-, C')- In Asplanchna (6) the stomach ends blindly, the 

 intestine, cloaca, and anus being absent. 



The excretory system is very uniform in structure. It con- 

 sists of a pair of more or less coiled nephridial tubes, placed 

 longitudinally and giving off lateral branchlets which end in 

 flame-cells. The outer surface of each flame-cell usually bears 

 one or sometimes two flagella, which lie free in the body-cavity. 



