18 



ZOOLOGY 



SECT. 



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of its ciliated cells drive 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 

 oesophagus. 



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 tentacles (Fig. 725, tent.). 



Enteric Canal. The ossophagus (Figs. 725 and 727, 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 is a 

 thickening the typh- 

 losole. 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 and are con- 

 nected with a duct 



FIG. 728. Ascidia, transverse section, bl. v. blood-vessels ; Opening into the 

 dors. lam. dorsal lamina; epi. epidermis; end. endostyle ; x rkrnQ ,,'U 4 Q 

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



to be of t 

 of a digestive gland. 

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

 (Fig. 727, 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 

 lining, and of spaces or lacunae, forming a haemoccele : in the 



