RENAL AND REPRODUCTIVE ORGANS. 17 



beneath the endostyle, giving off near its origin a branch to the 

 test (Fig. 12). The other after giving off a branch to the test, which 

 accompanies the branch from the endostylar vessel, is distributed 

 to the digestive viscera, gonads and body wall. The endo- 

 stylar vessel communicates with the sinuses in the pharyngeal 

 wall and these again with a dorsal vessel running along the 

 dorsal lamina. This posteriorly is distributed to the viscera and 

 body wall, whence the blood is returned to the posterior end of 

 the heart. It is a peculiarity very generally observable in 

 Tunicata that the heart contracts a certain number of times in 

 one direction and then a similar number of times in the opposite 

 direction. Thus for a certain number of beats it acts as a 

 respiratory heart driving the blood to the respiratory organs 

 and thence to the system. It then reverses its action and becomes 

 a systemic heart driving the blood first to the system and thence 

 to the respiratory organs.* 



The blood is colourless and contains nucleated corpuscles. 

 These are generally colourless and amoeboid, but some of them 

 generally contain pigment, either yellow, red, brown or blue. 



In the Botryllidae the terminal branches of the test-vessels are dilated 

 into ampulla-like sacs which have rhythmically contractile walls and assist 

 in the circulation of the colony. 



The renal organs t are but little understood and appear to 

 have no relation with the coelom. The only structures to which 

 a renal function has been ascribed are some vesicular bodies 

 containing concretions of uric acid and other substances, and 

 placed in the walls of the intestine, in the mantle and sometimes 

 in other places. These structures which have not been found 

 in all Tunicates are without a duct ; so that the excretory 

 matters cannot escape. In the Molgulidae there is a large 

 saccular body of this nature on the right side of the body. It 

 has been suggested that the neural gland plays a part in renal 

 excretion. 



Reproductive Organs. The Tunicata are with very few 

 exceptions (e.g. Oikopleura dioica) hermaphrodite, and as a 

 general rule the female organ ripens first. So far as can be 

 ascertained the gonads have no relations with the coelom, either 

 developmentaUy or otherwise. The glands are continuous with 



* Schultze, Jena Zeitsch., 35, 1001, p. 221 

 t Dahlgrtiru Arch. /. mik. Anat. 58. 1901, p. 608 

 Z III C 



