NOTE ON THE CILIATED PIT OP ASCTDIANS. 145 



It curves round the posterior end of the body, following the 

 line of the partition between the two halves of the atrial 

 cavity. For the greater part of its length it is situated in the 

 pseudocoele external to the atrial cavity, but the anterior end 

 of it lies in the partition, i. e. immediately ventral to the endo- 

 style. It extends backwards nearly as far as the oesophagus. 



At both ends it opens out into the pseudocoele, anteriorly by 

 a long narrow slit in its dorsal wall, and posteriorly by a 

 terminal pore, so that there is apparently no closed system of 

 blood-vessels, but the circulation is carried on only through the 

 blood-sinuses which surround all the organs and fill ail the 

 spaces usually occupied by the body cavity. 



The pericardium is entirely closed. 



The same condition of a heart situated in a closed peri- 

 cardium, and itself opening at both ends, is said by Seeliger 1 

 to exist in Clavellina. The gradual shutting off of the heart 

 from the body cavity in the development of the embryo, as he 

 describes it, explains how the adult condition may have been 

 brought about in the case of Cynthia. 



The Generative Organs. 



The generative organs are unpaired, and lie near the posterior 

 end of the body in the pseudocoele, outside the right half of 

 the atrial cavity (fig. 12, O. and ts.). 



They are arranged in two long masses on the outer and inner 

 sides of a cavity (fig. 14, G. D.), which is lined by flat cells, 

 and opens posteriorly into the atrial cavity. The mass on the 

 outer side consists of testes (figs. 12 and 14, ts.), that on the 

 inner of ova (figs. 12 and 14. 0.). 



The testes (fig. 13) are large oval sacs rilled with spermatozoa 

 (fig. 13, sp.) ; each sac is lined by a thin membrane, outside 

 which are bundles of muscles (M. ts.), and, especially at its 

 outer end, large collections of pigment-cells. At its inner end, 

 i. e. towards the cavity, it is drawn out into a short duct (figs. 

 13a, 14, D. ts) lined by low columnar cells, which terminates 



1 Seeliger, Oswald, " Die Entwicklungsgeschichte der Socialen Ascidien," 

 ' Jenaische Zeitschrift fur Naturgewissenschaft,' 1885. 



VOL. XXV1I1, PART 1. NEW SER, K 



