210 PROFESSOR HUXLEY ON THE ANATOMY 
of the mid-atrium ; but the testis is to the right, the ovisae to the left. The size of both 
these organs has a definite relation to the age and advancement in development of the 
ascidiozooid 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 young 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 Pyrosoma (§ 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 haemal 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 haemal 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 caeca are -g^th of an inch long, or thereabouts. 
Each consists of a delicate structureless membrana propria investing an aggregation of 
spheroidal corpuscles about ^fogth of an inch in diameter. Near the attached end of each 
caecum 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 T^th 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 lip, none 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 stolon-like diverticula in his pi. 22. fig. 1, 1, and he speaks of them in the "Systeme 
des Aseidies," p. 208, where, in characterizing the test of Pyrosoma giganteum, he says that it generally presents few 
vessels, " except in the diaphragm of the opening." He appears not to have been acquainted with the origin of these 
"vessels." In describing his variety c of this species, he states (supra) that the opening was surrounded by animals 
which were almost all adult. 
