XIII 



PHYLUM CHORDATA 



19 



perl.br 



these forwards to the peribranchial groove, around which they 



pass to the dorsal lamina, and the cilia on the cells of the latter 



drive them backwards to the opening of the oasophagus. 



Some little distance in front of the anterior peripharyngeal 



ridge, at the inner or posterior end of the oral siphon, is a circlet 



of delicate ttntacles (Fig. 716, tent.). 



Enteric Canal. The oesophagus (Figs. 716 and 718, 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- gn, va. a .tj* 



hand side. The 



stomach is a large 



fusiform sac with 



tolerably thick 



walls. The intes- 

 tine 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 

 typhlosole. There is 

 no liver ; but the 

 walls of the stomach 

 are glandular, and 

 a system of deli- 

 cate tubules which 

 ramify over the 

 wall of the intestine 

 and are connected 



with a duct opening into the stomach, is supposed to be of the 

 nature of a digestive gland. 



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

 (Fig. 718, ht.) is a simple muscular sac, situated near the stomach 

 in the pericardium a cavity entirely cut off from the surrounding 

 spaces in which the blood is contained. Its 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, when the con- 

 tractions begin again, they have the opposite direction. Thus the 

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

 regular intervals. There are no true vessels, the blood circulating 

 through a system of channels or sinuses devoid of epithelial 



c 2 



PIG. 719. Ascidia, transverse section, bl. v. blood-vessels ; 

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

 gn. ganglion ; hyp, neural gland ; mus. muscular layer of wall 

 of body ; peribr. peribranchial cavity ; ph. pharynx ; test, test ; 

 vas. tr. vascular ti-abeculse. (After Julin.) 



