588 



CHAETOGNATHA. 



The alimentary canal is a straight tube, and is divided into 



oesophagus and intestine. There 

 are no special glands opening 

 into it. 



The nervous system consists of 

 a cerebral ganglion in the head, 

 and a large elongated ventral gan- 

 glion the suboesophageal ganglion 

 placed in about the middle of 

 the body length. These two gan- 

 glia are connected by long circum- 

 oesophageal commissures, and the 

 whole of this part of the nervous 

 system lies in the ectoderm (Fig. 

 470). There is in addition a pair 

 of ganglia connected with the cere- 

 bral, and with each other below the 

 oesophagus ; these are placed in the 

 mesoderm and supply the muscles 

 of the head. 



The sense - organs consist of a 

 pair of eyes on the dorsal surface 

 of the head, innervated from the 

 cerebral ganglion ; the annular tract 

 of ciliated ectoderm on the dorsal 



surface already mentioned it is supposed to be olfactory in function, 



and varies in shape in different 



species ; and tactile papillae consisting 



of ectoderm cells, and distributed all 



over the surface. 



There is no vascular system. 

 The body-cavity is well developed, 



and, as is shown by its development 



and its relation to the male generative 



organs, is a coeloni. It is lined by 



a layer of flat epithelium, and is 



divided into two lateral halves by a 



longitudinal septum, in which the 



alimentary canal is placed (Fig. 470) ; 



this constitutes the dorsal and ventral 



mesentery, and in the tail the septum. 



B 



Fio. 469. Head of Sagitta bipimctata. A 

 dorsal, I! ventral view. <? 2 cerebral 

 ganglion ; kk cephalic hood ; re 1 circum- 

 oesophageal commissures ; no optic 

 nerve; nr olfactory nerve; o mouth; 

 r olfactory organ ; v hooks ; z spines. 

 (After Hertwig.) 



mes 



FIG. 470. Transverse section through 

 the trunk of Sagitta (from Lang, after 

 Hertwig). Ih coelom ; mes mesentery; 

 mil intestine ; Im longitudinal mus- 

 cles ; bg suboesophageal ganglion. 



