16 



ZOOLOGY 



SECT. 



bl.u 



-per ti 



lest 



the gland cells of the enclostyle forms viscid threads which entangle 

 food-particles (microscopic organisms of various kinds) ; the cilia 



of its ciliated cells 

 drive these for- 

 wards to the peri- 

 branchial groove, 

 around which they 

 pass to the dorsal 

 lamina, and the cilia 

 of the cells of the 

 latter drive them 

 backwards to the 

 opening of the oeso- 

 phagus. 



Some little dis- 

 tance in front of 

 the anterior peri- 

 pharyngeal ridge, 

 at the inner or pos- 

 terior end of the 

 oral siphon, is a 

 circlet of delicate 

 tentacles (Fig. 674 

 tent.). 



Enteric Canal. 

 The oesophagus 

 (ces.) leads from the 



pharynx (near the posterior end of the dorsal lamina) to the 

 stomach (stom.) which, together with the intestine, lies embedded 

 in the mantle on the left-hand side. The stomach is a large 

 fusiform sac with tolerably thick walls. The intestine is bent 

 round into a double loop, and runs forwards to terminate in an 

 anal aperture (an.) situated in the atrial cavity. Along its inner 

 wall runs a thickening the typlilosole. There is no liver ; but 

 the walls of the stomach are glandular, and a system of delicate 

 tubules which ramify over the wall of the intestine is supposed 

 to be of the nature of a digestive gland. 



The Ascidian has a well-developed blood system. The heart 

 (Fig. 676, Jit.) is a simple muscular sac, situated near the stomach 

 in a pericardium forming part of the primitive coelome. It's mode of 

 pulsation is very remarkable. The contractions are of a peristaltic 

 character, and follow one another from one end of the heart to the 

 other for a certain time ; then follows a short pause, and, wher the 

 contractions begin again, they have the opposite direction. Thus- 

 the direction of the current of blood through the heart is reversed 

 at regular intervals. At each end of the heart is given off a large 

 vessel. That given off ventrally, the Iranchio- cardiac vessel (br. car.), 



e-pt. 



FIG. 677. Ascidia, transverse section, bl. r. blood vessels ; 

 dors. lam. dorsal lamina; epi. epidermis; end. endostyle ; 

 C/n. ganglion ; hyp. hypophysis ; mus. muscular layer of wall 

 of body; peribr. peribraiichial cavity; ph. pharynx; test. 

 test ; xas. tr. vascular trabeculse. (After Julin.) 



