362 TUNICATA. 



Vascular System. — The tubular heart is a dilatation of a 

 ventral vessel, and lies in a pericardial space — part of the 

 body-cavity — to the side of the lower end of the pharynx. 

 For a number of beats it drives the blood upwards, and then 

 the direction is reversed. The same reversal occurs in 

 Phoronis. Transverse and longitudinal blood-vessels (they 

 cannot be called veins or arteries) lie on the walls of the 

 pharynx between the slits. 



According to Herdman, the ventro-dorsal contractions 

 occasion the following circulation. From the pharynx or 

 branchial sac, the blood collects in a branchio-cardiac vessel, 

 receives contributions from the tunic, enters the heart, is 

 propelled into cardio-visceral vessels which lead it to the 

 tunic, the viscera, etc., whence it is gathered in viscero- 

 branchial vessels and diffused on the branchial sac. When 

 the dorso-ventral contractions begin, all this is reversed. 

 The blood seems very colourless, but there are usually a 

 few pigmented corpuscles in it. 



Excretory System. — In the loop of the intestine, there 

 lies a mass of clear vesicles containing uric acid and other 

 waste-products. This seems to be a renal organ, in which 

 Bacteria are usually found, but there is no duct. Perhaps 

 the sub-neural gland has some renal function, as has also been 

 suggested in regard to the proboscis gland of Balanoglossus. 



Reproductive System. — Tunicates are hermaphrodite. The 

 simple reproductive organs are embraced by 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 out- 

 wards 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 alongside the oviduct. But 

 in many cases (where the reproductive organs are near 

 the cloaca) there are no ducts. The ova are surrounded by 

 follicular cells ; they seem to be fertilised within the body. 



Development. — In 1866, Kowalevsky traced the develop- 

 ment of Amphioxus ; he showed moreover that its earlier 

 stages were exactly paralleled by those of an Ascidian. 



The fertilised ovum divides completely. The result of 

 segmentation is a blastosphere. This undergoes regular 

 segmentation, and becomes a two-layered gastrula. 



Along the dorsal median line of this gastrula, the ectoderm 



