464 



ZOOLOGY 



SECT. 



invagination of the ectoderm, and therefore correspond to a 

 stomodceum. The muscular pharynx is absent in some of the Crypto- 

 cephala : when present it is frequently protrusible to a greater or 

 less extent (see Figs. 355, 371) ; 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 

 oesophagus, which may be dilated behind into a crop. The intestine 

 is nearly always more or less deeply constricted inter-segmentally, 

 and in the Aphroditea, or " Sea-mice " (Fig. 380), 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 ccBca which are to a greater or less extent 

 branched at their extremities. In the Hesio- 

 nida a pair of caeca which open into the 

 anterior part of the intestine frequently con- 

 tain gas, and probably have a hydrostatic 

 function. In some of the terrestrial Oligo- 

 chaeta (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 Chaeto- 

 poda its cells enlarged and granular to form 

 the so-called chloragen cells, which probably 



;, mouth - have an excretory function. The enteric epi- 

 b, pharynx; c, branching tlielium is very generally ciliated ; it contains 



cseca oi intestine ; d, anus. inn mi 



(From Gegenbaur's Cam- numerous gland-cells. Ihe stoniodaeum and 



the proctodaeum are lined internally by a 



cuticular layer, which is continuous 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 mesentery running to the alimentary canal from the 

 dorsal body-wall ; 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 is the case (the Glyceridce among the Phauero- 

 cephala), 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. 36). In the 

 majority of the Chaetopoda there is a highly developed vascular 



