ASCIDIAE COMPOSITAE 



8l 



groups of the ascidiozooids of a colony are known as " systems " 

 or coenobia (see Fig. 44, D ; also Fig. 53, p. 89). 



The Ascidiae Compositae may be divided into seven families, 

 which seem to fall into two well-marked sets: (1) MEKOSOMATA, 

 in which the heart and alimentary and reproductive viscera are 

 placed behind the branchial sac, so as to constitute a more or 

 less extended body divided into at least two regions (Fig. 46, B), 

 and sometimes three (Fig. 46, C) thorax, abdomen, and post- 



FIG. 45. Vertical section through a small part of a compound Ascidian colony. Asc. 1 

 and Asc. 2, Parts of two ascidiozooids whose cloacas (cl) open into the common 

 cloacal cavity (c.c) of the colony ; at.l, atrial lobes ; t, t, t, common test of the 

 colony. The structure of the posterior parts of the ascidiozooids would depend upon 

 the family (see Fig. 46). The arrows show the direction of the water currents. 



abdomen ; and (2) HOLOSOMATA, in which the body of the 

 ascidiozooid is short, compact, and not divided into regions (Fig. 

 46, A). The latter group comprises the two families Botryllidae 

 and Polystyelidae, which agree both in points of structure and in 

 having the same type of budding, and are probably derived from 

 ancestral Cynthiidae amongst Simple Ascidians ; while the Mero- 

 somata seem more nearly related to the Clavelinidae. 



Gemmation takes place in the Compound Ascidians in a 

 variety of ways, being sometimes very different in its details in 

 closely allied forms. There are, however, two main types of 

 budding, to one or other of which most of the described methods 

 may be referred. . These are : 



1. The STOLONIAL, or " epicardiac " type seen in the Mero- 

 somata, typically in Distomatidae and Polyclinidae, and compar- 

 able with the gemmation in Clavelinidae, Pyrosomatidae, and 

 Thaliacea outside this group. 



VOL. VII G 



