n8 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



pharynx to some considerable distance and so give to 

 it the function of a proboscis. 



The region of the crop of the earthworm is, in the 

 leech, specially modified in relation to the blood-suck- 

 ing habits of that form ; from either side it gives off 

 as many as eleven tubes or blind diverticula, which 

 occupy a very large proportion of the body cavity, and 

 appear to serve as strainers of the watery portion of 

 the blood which is pressed out through their walls. 

 The development of c?eca is not, however, confined to 

 the leech, for it is found also in the sea-mouse (Aphro- 

 dite), where the very numerous cseca are branched 

 towards their free ends ; in many other marine 

 worms the intestine has a more or less sacculated 

 appearance, owing to the tube being constricted at the 

 points where the septa between the body-segments are 

 developed. 



The Crepliyrea contrast strongly with the Annu- 

 lata so far as the arrangement of their intestine is 

 concerned, for this, in place of being straight, is 

 ordinarily coiled, and the anal opening is often found 

 within the limits of the anterior third of the body. 

 The most anterior portion of the tract has here, again, 

 the function of a proboscis, and is sometimes sur- 

 rounded by retractile tentacles ; in Bonellia, a form 

 which in adult life lives in mud or shells, the proboscis 

 is of great length, and is divided into two lobes at its 

 free end ; along the ventral surface of this organ there 

 runs a ciliated groove which reaches to the mouth, 

 and the whole apparatus is capable of being retracted 

 with great rapidity. 



The Rotatoria obtain their food from the cur- 

 rents of water which are set in motion by the cilia on 

 their " wheel-organ " or disc ; and comminute it by 

 means of a system of hard parts which is placed in an 

 anterior enlargement of the intestine, and consists 

 typically of two hammer-like pieces which are set 



