VI 



PHYLUM NEMATHELMINTHES 



317 



which are striated and disposed longitudinally in four Lands two dorso-lateral 

 and two-ventro-lateral an arrangement which recalls that of the corresponding 

 layer in Nematoda. 



Enteric Canal. --The mouth (Fig. 255, m.) is a longitudinal slit-like aperture 

 on the ventral surface of the head ; on either side of it are several sickle-shaped 



d.ejatfvnt 



m^ JL -^ 



Fir;. 256. Sagitta bipunctata. Transverse sections, A, of trunk ; B, of tail. coel. coelome ; 

 cvel. ept/im. layer of nuclei of the muscle-cells formerly regarded as a coelomic epithelium ; 

 (1. epthm. deric epithelium ; /. fin ; int. intestine ; m. muscles ; ovij. ovary ; s. testis. (After 

 Hertwig. ) 



chitinoid hooks (Fig. 257 gli-} which are moved by muscles in a horizontal plane 

 and serve as jaws. The anterior region of the head also bears spines, and is 

 strengthened by chitinoid plates and partly covered by a hood-like fold of the 

 integument. 



The mouth leads by a muscular pharynx or stomodseum into a straight intes- 

 tine (rf), which extends through the trunk 

 and opens by the anus (a) at the junction 

 of trunk and tail. 



Coelome. At the j auction of the head 

 with the trunk, and of the trunk with 

 the tail, are transverse partitions or 

 septa, dividing the coelome into compart- 

 ments. The trunk region of that cavity 

 is further sub-divided by two longitudinal 

 partitions, the dorsal and ventral mesen- 

 teries, which respectively connect the 

 dorsal and ventral surfaces of the intes- 

 tine with the body-wall : the tail-region 

 of the coelome is similarly divided into 

 right and left chambers by a longitudinal 

 vertical partition (Fig. 256, A and B). 



There is no trace of vascular system 

 or of excretory canals. The nervous 

 system, on the other hand, is much 

 better developed than in either of the 



preceding classes, in accordance with a free life and active movements. On 

 the dorsal side of the pharynx is a comparatively large brain (Fig. 257, </), which 

 sends off on each side a long nerve-cord, the cesophageal connective (sc.). The two 

 connectives sweep round the enteric canal and unite on the ventral surface, not 



/tf 



FIG. 257 Head of Sagitta bipunctata, 



from above, an. optic nerve ; au. eye ; 

 g. brain : gh. hooks ; rn. olfactory nerve ; 

 ro. olfactory organ ; sc. oesophageal 

 connective. (From Lang's Comparative 

 Anatomy, after Hertwig.) 



