436 



ZOOLOGY 



SECT. 



Polychseta ; when present it is frequently protrusible to a greater 

 or less extent (see Fig. 336); around its extremity, when it is 

 fully^protruded, are to be seen a circlet of papillae in some forms ; 

 and in many one or more horny teeth, situated in its interior, are 

 brought into play. A gizzard with thick walls may follow upon 

 this protrusible pharynx, sometimes preceded by an oesophagus, 

 which may be dilated behind into a crop. The intestine is nearly 

 always more or less deeply constricted between the segments, and 

 in the Aphroditea, or Sea-mice (Fig. 343), there are in each of the 

 segments (with the exception of one or two of the most anterior 

 and one or two of the most posterior) a 

 pair of cceca which are to a greater or less 

 extent branched at their extremities. In 

 the Hesionida and Syllida a pair of caaca 

 which open into the anterior part of the 

 intestine frequently contain gas, and prob- 

 ably have a hydrostatic function. In some of 

 the terrestrial Oligochaeta (Earthworms) a 

 fold of the intestinal wall, the typhlosole, 

 projects into its lumen. The intestine is 

 straight in most, but is somewhat coiled in 

 the Chlorcemidce, Sternaspis, and others. The 

 wall of the alimentary canal consists (1) of 

 the visceral layer of peritoneum ; (2) of 

 longitudinally arranged muscular fibres : 



(3) of circularly arranged muscular fibres ; 



(4) of enteric epithelium. The peritoneum 

 on the surface of the intestine has in many 

 Chastopoda its cells enlarged and granular 

 to form the so-called chloragen cells, which 

 probably have an excretory function. The 

 epithelium is very generally ciliated ; it con- 

 tains numerous gland-cells. In addition the 

 stomodaBum and the proctodasum are lined 

 internally by a cuticular layer, which is con- 

 tinuous with the cuticle of the general surface. The anus is 

 usually terminal in position, sometimes directed towards the 

 dorsal aspect. There is, in most instances, a longitudinal mesen- 

 tery running to the alimentary canal from the dorsal body- wall ; 

 and sometimes a ventral mesentery is also present bearing a 

 corresponding relation to the ventral surface. 



Some Chsetopoda are entirely devoid of blood-vessels. In one 

 family in which this occurs (the Glyceridce among the errant Poly- 

 chseta), the perivisceral fluid, which assumes some of the functions 

 of the blood, contains numerous red corpuscles, the red colour of 

 which is due to the presence of haemoglobin (see p. 34). In the 

 majority of the Chastopoda there is a highly developed vascular 



FIG. 343. Enteric canal of 

 Aphrodita. , mouth ; 

 b, pharynx ; c, branching 

 cceca of intestine; d, anus. 

 (From Gegenbaur's Com- 

 parative Anatomy.) 



