344 Transactions. — Zoology. 



The dorsal tubercle is very complicated, " apparently by 

 the development of lateral branches from the original slit '' 

 (HerdmaTi) . 



G. Atrium. 



The atrium, or space surrounding the branchial sac, 

 and communicating with the exterior by the atrial aper- 

 ture, presents no essential differences from that in other 

 Ascidians. The atrial aperture, which lies, as before stated, 

 nearer the posterior end of the body, terminates a short tubular 

 process of the mantle, and the atrial siphon is provided with 

 a sphincter muscle, a lining prolongation from the test, and a 

 partial diaphragm at its lower end. 



The peribranchial cavity encircles the branchial sac except 

 along its ventral edge, where the two lateral halves of the 

 cavity are separated by the union of the endostyle with the 

 mantle. The cavity is crossed by blood-vessels connecting 

 the branchial sac with the sinuses of the mantle. Besides 

 these, the branchial sac is united to the mantle by the oeso- 

 phagus along the whole length of the endostyle (except at its 

 posterior end), round the anterior end at the peripharyngeal 

 bands, and along the first portion of the dorsal lamina. 



The atrium is in free communication with the interior of 

 the branchial sac through the stigmata, and is traversed by the 

 water in its course to the atrial aperture. 



The anus and genital ducts open into the peribranchial 

 cavity in the dorsal median region, often called the cloaca. 



Histology. 

 Histology of Test. 



The matrix of the test, which is for the most part homo- 

 geneous, is, however, fibrillated in some places. Immediately 

 beneath the external surface are round masses of various sizes. 

 Eouud them lie, thickly scattered, ordinary cells, rounded 

 in appearance. The rounded masses lie in a single row, 

 either closely packed or having short intervals between them. 

 I have not been able to make out their exact structure, but 

 they appear to be formed of a large number of separate cells. 



The small blood-vessels, with their terminal dilatations, 

 which I described, in connection with the vascular system, as 

 ramifying through the test, are lined by a layer of epithelium. 



Histology of Endostyle. 

 The endostyle is in the form of a groove, bounded by 

 parallel lips of considerable height, and projecting into the 

 interior of the branchial sac. The groove is lined by a modi- 

 fication of the epithelium of the interior of the branchial sac. 

 The branchial epithelium is continued on the lips of the groove 



