MOSS-POLYPES AND LAMP-SHELLS 435 



opens into a mouth-cavity <md this again into a muscular pharynx, 

 which is continued by a short gullet into a wide stomach, this 

 narrowing into an intestine which opens to the exterior on the 

 upper surface of the body. Special interest attaches to the 

 pharynx, usually known as the mastax, for the lining of this is 

 thickened into a complicated set of hard pieces which work upon 

 one another to crush the food. The transparency of the animal 

 permits this process to be clearly seen, and it presents a curious 

 and interesting spectacle, reminding one of the gastric mill in a 

 lobster or the gizzard of a bird. The last section of the intestine 

 is a cloaca, into which opens a thin-walled bladder (contractile 

 vesicle) receiving nitrogenous waste matter from a couple of 

 branched excretory tubes, which in some respects resemble the 

 corresponding structures of the unsegmented worms. There are 

 no specialized organs of circulation, these being represented 

 simply by a fluid - containing body - cavity. Respiration takes 

 place by the general surface of the body, as is very commonly 

 the case with minute animals of all grades. The nervous system 

 consists of a comparatively large brain, placed in the front part 

 of the body above the mouth. A red eye-spot is imbedded in its 

 upper surface, and it is also connected with a stiff dorsal tentacle 

 (calcar) which ends in a bunch of stiff hairs and is probably an 

 organ of touch. 



Rotifers differ from one another in many respects. The 

 general outline of the body varies largely, and the wheel-organ 

 in particular may be very variously shaped. Sometimes the 

 cuticle is thickened into a firm protecting carapace. Many 

 members of the group are fixed, and in this case a cement-gland 

 opens on the tip of the tail. Such forms often live in a cup or 

 " house " of various nature. Some of these will be described 

 later on, when the defences of animals are dealt with in detail. 



MOSS-POLYPES AND LAMP-SHELLS (MOLLUSCOIDA) 



Considerable doubt attaches to the affinities of the two aquatic 

 groups here associated together for convenience sake. They 

 were originally included in the Mollusca, and at a later date 

 Huxley established a sub-kingdom Molluscoida in which they 

 and the Tunicates were placed. When, however, it was proved 

 beyond doubt that the last group should without doubt be asso- 



