PHYLUM OHORDATA 



19 



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these forwards to the peribranchial o-roove, around wliieh tlie}' 

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

 drive them backwards to the openin^^- of the ovsophao-us. 



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 Untadcs (Fig. 71G, tent.). 



Enteric Canal. — The oesophagus (Figs. 716 and 718, as.) leads 

 from the phar\-nx (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 largo 

 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 



G 2 



'Perihr 



test 



hi. 



blood-vcsscls ; 

 end. endostyle ; 



in. 710. — Ascidia, transverse section. 

 doi-s. Jam. dorsal Llmina ; e'pi. epidermis, 

 rin. ganglion ; luip, neural gland ; 7n«."!. muscular layer of wall 

 of body ; periljr. poribrancliial cavity ; pA. pharynx ; test, test 

 I'as. fr. vascular trabcculie. (After Julin.) 



