308 



ZOOLOGY 



SECT 



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

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

 very slender, the incus assuming the form of forceps (Fig. 251, 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) 



B 



y 



FIG. 251. Typical forms of mastax. A, forcipate type ; B, incudate type ; C, ramate type 

 /. fulcrum ; m. manubrium ; /. ramus ; c.. 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). 



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 defalcation 

 (Fig. 249, 4> ft )- I n -Asplanchna (&) 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. Frequently, but not always, the two tubes open 

 posteriorly into a contractile vesicle or bladder which discharges 

 into the cloaca. 



Nervous System and Sense Organs. The nervous system 

 always consists of a single ganglion (Fig. 248, br) towards the 

 dorsal aspect of the anterior part of the body, and representing the 

 brain or supra-oesophageal ganglion of the higher Worms : it sends 

 nerves to the muscles, trochal disc, and tactile organs. One or 

 more eye-spots (e) are usually present, and are always mere spots 

 of pigment in close relation with the brain. The only other organs 

 of sense are the tactile rods (d.f., If), of which there is usually one 

 on the dorsal surface near the anterior end of the body, and 

 frequently two others, one on each side of the trunk. They are 



