PHYLFM ANNULATA 



475 



segments. The septa are not complete partitions, there being 

 always apertures of greater or less extent by which the 

 cavities of neighbouring segments communicate. The septa 

 consist of double folds of the peritoneum enclosing muscular 

 fibres. 



The enteric canal is an elongated, and nearly always straight 

 tube, running through the entire length of the body from mouth 

 to anus. A number of different parts are usually distinguishable : 

 but their disposition varies to a very great extent in the different 

 groups. The buccal cavity, into which the mouth leads, is followed 

 by a muscular pharynx ; these are both formed in the embryo by 

 invagination of the ectoderm, and therefore 

 correspond to a stomodceum. The muscular 

 pharynx is absent in some of the Cryptoce- 

 phala : when present it is frequently pro- 

 trusible to a greater or less extent (see 

 Figs. 349, 365) ; 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, and is sometimes preceded by an 

 (esophagus, which may be dilated behind into 

 a crop. The intestine is nearly always 

 more or less deeply constricted in each seg- 

 ment, and in the A'phroditea, or " Sea-mice " 

 (Fig. 374), 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 caeca which are to a 

 greater or less extent branched at their 

 extremities. In the Hcsiomda and Syllida 

 a pair of cseca which open into the anterior 

 part of the intestine frequently contain gas, 

 and probably have a hydrostatic function. 

 In some of the terrestrial Oligochseta (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 

 ChlorcemidcB, Sternapsis, and others. The wall of the aliment- 

 ary 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 man}' Chsetopoda 

 its cells enlarged and granular to form the so-called chloragen 

 cells, which probably have an excretory function. The enteric 

 epithelium is very generally ciliated ; it contains numerous gland - 



Fjn. 374.— Enteric canal of 

 Aphrodite, a, mouth ; 

 b, pharynx ; c, branchinjj 

 caeca of intestine ; d, anus. 

 (From Gegenbaur's C(y^n- 

 iMi'ative Anatomy.) 



