210 PROFESSOR HUXLEY ON THE ANATOMY 



of the mid-atrium ; but the testis is to the right, the ovisac to the left. Tlie size of both 

 these organs has a definite relation to the age and advancement in development of the 

 aseidiozooid in which they occur, being larger and more advanced as it is older and 

 nearer perfection. Hence the early stages of both ovisac and testis can only be observed 

 in buds and yonng ascidiozooids. The generative blastema will be most conveniently 

 considered in connexion with the process of gemmation and in describing the most 

 advanced condition of the foetus. The description of the ovisac will form the most fitting 

 commencement of the history of sexual propagation in JPyrosoma (§ 3) ; all that 

 remains, therefore, is to give in this place some account of the structure of the testis 

 and of the character of its products. 



The testis lies in the hgemal sinus above the mid-atrium, and on the right side of the 

 ovisac. It consists of about a dozen cylindroidal caeca, free at their neural ends, but 

 connected at their hsemal extremity with the dilated upper end of a vas deferens, which 

 passes directly to the neural side and somewhat backwards, to open by a slightly raised 

 papilla on the roof of the mid-atrium. The cajca are -i^^ih. of an incli long, or thereabouts. 

 Each consists of a delicate structureless membrana propria investing an aggregation of 

 spheroidal corpuscles about 47^g^th of an inch in diameter. Near the attached end of each 

 csecum the rod-like heads of the spermatozoa become visible, and gradually take the place 

 of the spheroidal cells. The duct has the same structure as the caeca. It presents an 

 upper and a middle dilatation, but is not more than Teoth of an inch wide at its termi- 

 nation. The middle dilatation is usually full of closely packed spermatozoa. 



The structure which has been described is characteristic of any of the fully-formed 

 ascidiozooids in the middle of the ascidiarium, or towards its apical end, in which regions 

 the number of such ascidiozooids bears a large ratio to the total. 



But towards the open end of the ascidiarium fully-formed ascidiozooids become scarcer 

 and scarcer, until, close to the inflected cloacal Hp, noiie are discernible. On the other 

 hand, all those ascidiozooids which are to be found in this region possess an appendage 

 which is not to be discovered in the others, in the shape of a long tubular diverti- 

 culum of the external tunic, or stolon, which extends from the neural side of the body, 

 behind the oesophageal aperture, into the lip of the cloaca, at whose free edge it ends 

 in a ctecum. The walls of these diverticula, composed of the external tunic only, 

 exhibit strongly marked parallel longitudinal striae, as if they were composed of mus- 

 cular fibrillae*. 



The test. — The common integument, or test, in which all the ascidiozooids are enclosed, 

 appears to the naked eye to be quite glassy and homogeneous ; but when thin sections 

 taken in various directions are submitted to the microscope, it is found to possess marked 

 structural peculiarities. Dispersed through its general substance are numerous cells 

 with radiating processes, like connective-tissue corpuscles, and containing a central endo- 



* Savigny has figured these stolou-hke diverticula in his pi. 22. fig. 1, 1, and he speaks of tliem in the " Systeme 

 des Ascidies," p. 208, where, in characterizing the test of Pyrosoma yiganteum, he says that it generally presents few 

 vessels, " except in the diaphragm of the opening." He appears not to have heen acquainted with the origin of these 

 "vessels." In describing his variety e of this species, he states (sujmi) that the opening was surrounded by animals 

 which were almost all adult. 



