STRUCTURE OF ASCIDIAN 505 



It is probable that the pigment spots between the lobes of the 

 apertures, the tentacles in the branchial siphon, and the dorsal lamina, 

 or its representatives, the languets, have some sensory function. 



Vascular system. — The simple tubular heart lies in a 

 pericardial space at the ventral side of the lower end of the 

 pharynx. In development, two diverticula grow out from 

 the pharynx ; these meet and fuse, forming the peri- 

 cardium. The heart arises as an invagination from its 

 dorsal wall, and is thus endodermal in origin, and probably 

 not homologous with the heart of the other Vertebrates. A 

 periodical reversal of the direction of the waves of con- 

 traction is discernible in the heart ; for a certain number of 

 beats the blood is driven upwards, and then the direction 

 is reversed. This same reversal also occurs in Phoronis. 



According to Herdman, the ventro -dorsal contractions occasion the 

 following circulation : — The blood, which is spread out on the walls of 

 the pharynx in vessels lying between the slits, collects into one large 

 (branchio-cardiac) vessel, which, after receiving a vessel from the test, 

 enters the ventral end of the heart. From the dorsal end it is poured 

 into a great (cardio-visceral) trunk, which sends one branch to the 

 test, and then breaks up among the viscera. From the visceral lacunae 

 the blood is collected (in a branchio -visceral trunk) and distributed 

 to the branchial sac. At the reversal of the contractions this circulation 

 is also reversed. The reversal occurs every couple of minutes or so. 

 The blood is very colourless, but usually contains a few pigmented 

 corpuscles. 



Excretory system. — In the loop of the intestine there 

 lies a mass of clear vesicles containing uric acid and other 

 waste products. This, therefore, seems to be a renal organ, 

 but there is no duct. Bacteria are usually found in the 

 vesicles, and their activity may make diffusion easier. It is 

 interesting to find such a plant-like method of storing up, 

 instead of eliminating, waste products in these very passive 

 animals. It has been suggested that the sub-neural gland 

 may have some renal function. 



Reproductive system. — Tunicates are hermaphrodite. 

 The reproductive organs (Fig. 285, G.) are very simple, and 

 lie in the loop of the intestine. The ovary is the larger, 

 and contains a cavity into which the ova are set free, and 

 from which they pass outwards along an oviduct which 

 opens into the cloacal chamber. The testis surrounds the 

 ovary, and is mature at a different time (dichogamy) ; its 

 duct runs by the side of the oviduct. In some forms, 



