12 Dr. Asajiro Oka: 



Discription of the Zooid. 



Form. As the zooids are attached to the common test rather 

 firmly at the branchial apertm'e and at certain regions of the 

 thorax, it is almost impossible to dissect out a zooid uninjured 

 from the investing mass. It invariably breaks in the middle of 

 the branchial region where it is weakest, since the body wall is 

 here very imperfectly developed and the delicate longitudinal 

 vessels of the branchial sac lie exposed on the outer surface. At 

 other places, however, the body wall seems to be only loosely 

 attached to the common test, since in sections of preserved speci- 

 mens empty spaces are commonly found surrounding the zooids, 

 apparently in consequence of contraction of the latter. 



The body of the zooid may be divided into two regions, the 

 thorax and the abdomen. They are about of the same size, and 

 the boundary between them is not very distinct. The thorax 

 comprises chiefly the branchial sac and its accessory organs, while 

 the abdomen is composed of the remaining parts of the alimentary 

 canal together with the heart and the reproductive organs. A 

 vascular appendage springing from the posterior end of abdomen 

 traverses the common test towards the base of the colony. The 

 zooids, without the appendage, are 3 — 4 mm. in length and 

 nearly half so much in breadth ; when fresh and fully expanded 

 they measured, of course, somewhat more. 



The thorax is roughly cylindrical in shape. The anterior end 

 is only slightly convex and hexagonal in outline, with the bran- 

 chial siphon placed at the centre. The side walls of the thorax 

 are very incomplete, as the external wall of the peribranchial 

 space is developed only on the ventral side. Dorsally as well as 

 laterally there is no body wall, and consequently the branchial 

 sac is quite exposed on these sides (Pl. I., fig. 3). This condition 

 may perhaps be considered as the result of the atrial aperture 

 having become disproportion ally widened so as to occupy the 

 gi'eater part of the surface of the thoracic region. As mentioned 

 before, there is only one species of compound ascidians in which a 

 similar condition is met with. Lahille, in his "Recherches 

 sur les Tuniciers " ('30) describes the thoracic region of the zooids 



