294 



ZOOLOGY 



SECT. 



chitmoid hooks (Fig. 241, yh.) which are moved l>y 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 parti}- covered by a hood-like fold of the 

 integument. 



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

 tine (d), which extends through the trunk and opens by the anus (a) at the junction 



d.ejblh/m 



FIG. 2-10. Sagitta bipunctata. Transverse .sections, A, of trunk ; B, of tail. cod. coelome ; 

 cod. epthm. coelomic epithelium ; (1. cptlaii. deric epithelium ;/. fin ; int. intestine ; rn. muscles ; 

 ovy. ovary ; ts. testis. (After Hertwig.) 



of trunk and tail. The wall of the intestine is made of two layers of cells an 



inner of columnar cells, the enteric epithelium ; and an outer of very delicate 



flattened cells, the coelomic epithelium. 



Coelome. At the junction 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 In- two longitu- 

 dinal partitions, the dorsal and ventral 

 mesenteries, which connect the dorsal 

 and ventral surfaces respectively of the 

 intestine with the body-wall. The form- 

 ation of the mesenteries is best seen in 

 a transverse section (Fig. 240, A), which 

 shows that at the middle dorsal line the 

 layer of coelomic epithelium lining the 

 body-wall (parietal layer] becomes de- 

 flected downwards, forming a two- 

 layered membrane, the dorsal mesentery : 

 the two layers of this, on reaching the 

 intestine, diverge and pass one on either 

 side of it, forming the visceral layer of 

 ccelomic epithelium : uniting again below 



i.'i< ; . 241. Head of Sagritta bipunctata, 



from above, an. optic nerve ; ni>. eye ; 

 g. brain ; (th. hooks ; rn. olfactory nerve ; 

 ro. olfactory organ ; sc. oesophageal 

 connective. (From Lang's Comparative 

 Anatomy, after Hertwig.) 



the intestine, they are continued down- 

 wards as the vciifrti.l im-stntery, and on 

 reaching the body- wall diverge once 



more to join the parietal layer. The tail-region of the coelome (B) is similarly 

 divided into right and left chambers by a longitudinal vertical partition. 



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 



