KEPOET ON THE THNICATA. 
71 
frequently it is a stalked club-sliaped mass, the upper swollen part of which may be short 
and globular (as in Colella gaimardi), or long and cylindrical (as in Colella thomsoni). 
In some cases systems with common cloacal openings are formed, while in others the 
Ascidiozooids are not arranged regularly, and each one has an atrial aperture communi- 
cating separately with the exterior. In the latter case the atrial aperture is six-lobed, 
in the former it has a large atrial languet. The branchial aperture is always six-lobed. 
The test varies greatly in its condition. It may be only moderately thick aud remain 
soft and gelatinous, or it may become greatly thickened, forming a peduncle or a very 
massive colony, and it may be very dense and cartilaginous, or may even develop 
calcareous spicules (as in the genus Cystodytes). 
The Ascidiozooids are always of the Distoma type, and would have been considered 
by Milne-Edwards as “ Didemniens.” They have a thorax or branchial region, and an 
abdomen or intestinal region; usually the two are separated by a deep constriction 
(see PI. XIV. fig. 5). There are general]}^ vascular appendages or ectodermal processes, 
which, springing from the posterior ends of the xiscidiozooids, penetrate the test for 
considerable distances (PL V. fig. 13, and PI. XIV. fig. 12, v.ap.), and may even branch 
to form networks like those met with in some Botryllidse (see PI. II. fig. 1, and 
PI. XVIII. fig. 9). 
The branchial sac varies greatly in size and number of stigmata. In Distoma mucosa^ 
von Drasche, there are only three rows of stigmata, while in Distoma adriaticum, von 
Drasche, there maybe as many as twenty-four. In some cases {e.g., Colella pedunculata,. 
Quoy and Gaimard) the stigmata may be very long and narrow, and more like those of 
some Simple Ascidians than of a Compound Ascidian. In Symplegma there are internal 
longitudinal bars, but in all other forms they are absent. 
The alimentary canal also presents various conditions in the family. It may be short 
and thick, or it may form a long loop upon which the stomach is always a conspicuous, 
organ. The wall of the stomach may be smooth or longitudinally folded or thickened 
irregularly. 
There are usually a large number of spermatic vesicles, which all communicate by 
small ducts with a large vas deferens. The ova form a mass placed upon the intestinal 
loop and usually projecting beyond it. In some cases [Distapdia and Colella) the 
embryos undergo their development in a special prolongation of the peribranchial 
cavity, the incubatory pouch. The mature ova and the embryos are often of very 
great size (PI. VII. figs. 7, 14, &c.). Gemmation takes place mainly, I believe, if not 
entirely, in connection with the vascular appendages. In some cases larval budding 
occurs. 
The various genera in this family may be shortly distinguished by the following 
diagnostic characters : — 
