294 



ZOOLOGY 



SECT. 



chitinoid 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 partly covered by a hood-like fold of the 



integument. 



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

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



d.efilfvm 



B .< 



=> _ W-U!ki. !/':, . 

 ccel 



'^m i n ^ 



^ 



Fit.. -40. Sagitta bipunctata. Transverse sections, A, of trunk ; B. of tail. co-:i. cn.-li.un.' : 

 ccel. < /''I, 1,1. ccelomic epithelium ; <L tptltm. cleric epithelium \f. fin ; i,>l. intestine ; m. muscles ; 

 or >i. ovary ; ?s. testis. (After Hertwig.) 



fVZ,- 



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 ccelomic epithelium. 



Ccelome. At the junction of the head with the trunk, and of the trunk with 

 the tail, are transverse partitions or .wjita, dividing the ccelome into compart- 

 ments. The trunk-region of that cavity 

 is further sub-divided by 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 ccelomic epithelium lining the 

 body-wall ()>ari<J.al lai/tr) 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 

 siik' of it, forming the rixceral layer of 

 ccelomic epithelium : uniting again below 

 the intestine, they are continued down- 

 wards as the rentrnf mesentery, and on 

 reaching the body-wall diverge once 



more to join the parietal layer. The tail-region of the ccelome (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 



FIG. -241. Head of Sagitta bipunctata, 



from above, an. optic nerve; c<i!. eye; 

 ;/. brain ; <ilt. hooks ; rn. olfactory nerve ; 

 ro. olfactory organ ; xr. cesophageal 

 connective. (From Lang's Comparativt 

 Aiuttoni,!, after Hertwig.) 



