vii PHYLUM TROCHELMINTHES 301 



Digestive Organs. The mouth (Fig. 248, 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 luccal 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. 246) of rounded form, and contains, as a thickening of its 

 cuticular lining, an elaborate 

 apparatus for triturating the 

 food. In the middle line is a 

 forked structure, the incus, 

 consisting of a small base 

 or fulcrum (/.) and of two 

 branches or rami (?.). On 

 either side of the incus is a 

 hammer-like structure, the 

 malleus, consisting of a handle 

 or manulrium (m.) and of a F[o 24f) ._ Pharyll!C of Bracblonus rabens . 



toothed, head Or UnCUS (U.). f. fulcrum ; m. manubrium ; u. uncus ; r. 



By means of the muscular 



walls of the chamber the 



heads of the mallei are worked backwards and forwards upon the 



forked incus, 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 stomodaeum, the cloaca from the proctodseum. Between the 

 body-wall and the enteric canal is a spacious coelome containing 

 a fluid which serves the purpose of blood and contains minute 

 granules. 



The excretory system consists of paired nephridial tubes 

 (Figs. 245 and 248, nph.) resembling those of the Platyhelminthes. 

 Their general direction is longitudinal, but they are a good deal 

 coiled and give off little tag-like processes ending in flame-cells. 

 The lumen of the tubes is intra-cellular : it is uncertain whether or 

 not the cavities of the flame-cells communicate with the coelome 

 by apertures in their walls. Posteriorly the nephridial tubes open 

 into a bladder or contractile vesicle (c. v.), the contents of which 

 are discharged, by periodical contractions, into the cloaca. 



Nervous System and Sense Organs. There is a single 

 ganglion or brain (br.), of proportionally large size, situated at the 



