REPORT ON THE TUNICATA, 
15 
have become retractor muscles for the purpose of pulling the Ascicliozooicls downwards 
into the test (as in the case of Leptoclinum thomsoni). 
In some of the Compound Ascidians certain parts of the test may become very greatly 
enlarged so as to produce large massive colonies (such as Atopogastev aurantiaca, PI. XXIIl. 
fig. 7; and fig. 5, p. 27), or long peduncles upon which the rest of the colony and the 
Ascidiozooids are borne {e.g., Colella pedunculata, PL V. fig. 1, and Distaplia vallii, 
PL XVIII. fig. 1, &c. ; see also fig. 6, p. 27). Usually in such cases the enlarged part 
of the test becomes modified. Peduncles, for example, are almost invariably harder 
and tougher than the rest of the test. The superficial layer of test also frequently 
becomes slightly modified to form a firmer layer which may be stripped off as a membrane 
from the surface of the colony. In some cases the outer layer of the test may be pro- 
longed into delicate processes for purposes of attachment (see PL XXXV. fig. 3), and such 
processes may become covered wuth adhering sand-grains so as to form a protecting 
sandy investment. 
The Ascidiozooid. 
The Ascidiozooids or members of the colony are usually placed vertically (at right 
angles to the upper surface) in the test. In the genus Botryllus, however, they are placed 
nearly horizontally, and in Diplosoma and some other forms they are inclined at all 
angles. The anterior end, indicated by the branchial aperture,^ is always nearest to the 
outer surface of the colony, and the antero -posterior axis generally runs along the length 
of the body (see fig. 3, p. 16). 
The Ascidiozooids vary greatly in shape in different groups of the Compound Ascidians. 
Savigny in 1816 pointed out and figured^ most of these shapes, and in 1842 Milne- 
Edwards ® arranged them in three groups, one characteristic of each of his great sections, 
Polycliniens, Didemniens, and Botrylliens. These divisions of the Compound Ascidians 
are no longer recognised, but still it is useful to employ Milne-Edwards’ arrangement so 
far as it relates to the shape of the Ascidiozooids. In his group Polycliniens the body 
consists of three regions placed one behind the other — first, the branchial or “ thorax ” 
(see fig. 3) ; second, the intestinal or abdomen ; and third, the reproductive or post- 
abdomen. The “thorax,” to make use of Milne-Edwards’ terms, contains the branchial 
siphon and branchial sac with all its accessory organs (endostyle, &c.), the nerve ganglion, 
and the terminal part of the rectum. The abdomen contains the alimentary canal from 
the oesophagus to the rectum, with any accessory digestive glands or cseca which may be 
present. The post-abdomen is composed of the reproductive organs — with the exception 
of the greater part of their ducts, which pass upwards into the abdomen and the thorax 
— and of the heart. From the posterior end of the post-abdomen occasionally one or 
1 See Part I. of this Report, p. 27. ^ M^moires, &c. ^ Observations, &c. 
