3360 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
a middle dilatation, but is not more than ;35th of an inch wide at its 
termination. 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 ascidio- 
zooids 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 
diverticulum of the external tunic, or stolon, which extends from the 
neural side of the body, behind the cesophageal aperture, into the lip 
of the cloaca, at whose free edge it ends in a cecum. The walls of 
these diverticula, composed of the external tunic only, exhibit strongly 
marked parallel longitudinal striz, 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 cor- 
puscles, and containing a central endoplast or nucleus. The cells 
measure on an average 4,\g9th of an inch in diameter; and their 
processes become very fine before they are lost in the surrounding 
nearly homogeneous matrix. In two regions this general structure is 
departed from. At the cloacal wall of the test, there lies immediately 
beneath the surface a thin film of reticulated tissue, consisting or 
cells similar in their essential structure to those just described, but 
set more closely, more granular, elongated, and united together by the 
coalescence of their processes. Again, in a plane which would 
correspond with the peripharyngeal ridges of the ascidiozooids, and 
therefore near the outer surface, the test exhibits a very faint longi- 
tudinal striation, as if it were fibrillated. 
1 Savigny has figured these stolon-like diverticula in his pl. 22. fig. 1, 1, and he 
speaks of them in the ‘‘Systéme des Ascidies,” 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 (s7éra) that the opening was 
surrounded by animals which were almost all adult, 
