430 SUB- PHYLUM UROCPIORDA OR TUN 1C AT A. 



already described, converge dorsally to form the dorsal lamina. In 

 Ascidia the sub-neural organ is ventral to the brain, and partly 

 glandular in character, and so it is in many ; in some cases, however, it 

 is dorsal in position, and its glandular portion is reduced to nil. It is 

 probable that the sub-neural gland and its duct correspond to the 

 olfactory pit oiAmphioxus, and perhaps to the hypophysis of Vertebrates 

 (see p. 468). 



It is further 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 pericardium. 

 The heart arises as an evagination from its dorsal wall, and 

 is thus endodermal in origin, and probably not homologous 

 with the heart of the other Vertebrates. According to some 

 authorities, the cavities of the heart and of the blood vessels 

 are blastoccelic in origin, i.e. they are said to be derived 

 from the segmentation cavity of the embryo. A periodical 

 reversal of the direction of the waves of contraction is 

 discernible in the heart ; for a certain number of beats the 

 blood is driven upwards, and then the direction is reversed. 

 This is said to be, at any rate partially, due to the differences 

 in oxygenation of the blood at the two ends of the heart. 

 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 

 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 trunk, 

 which sends one branch to the test, and then breaks up among the 

 viscera. From the visceral lacunas the blood is again collected, to be 

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

 circulation is also reversed. 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 



