254 
THE VOYAGE OF H.M.S. CHALLENGED. 
spicules. Ectodermal processes well developed, and provided with muscle 
fibres. 
Branchial Sac small, and not well developed. Eows of stigmata few, usually 
three or four. 
Alimentary Canal united to thorax by a narrow neck. Stomach usually smooth 
walled. 
Reproductive Organs placed alongside the intestinal loop. Male system consist- 
ing of a single large testis around which the first part of the vas deferens is 
spirally coiled. 
Gemmation from the pyloric region ; thorax and abdomen of the new Ascidiozooid 
formed from separate buds. Embryonic blastogenesis rudimentary only. 
This is a clearly defined family, most of the members of which have a very character- 
istic appearance, which distinguishes them from all other groups of Compound Ascidians. 
The colony is in most cases, like that of most of the Botryllidse, a flat expanded crust, 
which may be of any shape {Leptoclinum), and is usually of a pure white colour. In some 
cases, however, the colony becomes thickened to form a large rounded mass {Didem num). It 
is never mucli elongated vertically. Common cloacal apertures are usually distinctly visible, 
and they may be of large size, but the systems are always irregular and difficult to trace. 
The Ascidiozooids seem at first sight very like those of the Distomidse, and Milne- 
Ed wards classed the two groups together as “ Didemniens.” In both cases the body 
consists of two regions only, the thorax and the abdomen ; and in the Didemnidse, as 
in many Distomidae, these regions are separated by a very narrow neck composed of the 
oesophagus, the rectum, and the vas deferens, surrounded by a covering of mantle. A 
more minute examination of the Ascidiozooids shows, however, that in the details of 
structure the Distomidse and the Didemnidse are not really closely allied. In the latter 
group the branchial sac is feebly developed, and has only three or four rows of small 
stigmata, while in most of the Distomidse the branchial sac is large and well developed ; 
the mantle also is characteristic in the Didemnidse, and it gives off ectodermal processes 
or vessels which leave the body of the Ascidiozooid not at the posterior end of the body, 
as in most other Compound Ascidians, but near the anterior end of the branchial sac, 
where it joins the branchial siphon, and in the neighbourhood of the posterior end of the 
endo.style. These vessels always contain muscle fibres continuous with those of the 
mantle, and appear to act, as von Drasche suggests,^ as retractor muscles serving to 
change slightly the position of the Ascidiozooid in the investing mass. The vessels 
arising from the posterior end of the endostyle are provided with terminal knobs or bulbs. 
Tlie test or investing mass is usually gelatinous or cartilaginous, and its matrix is 
clear and structureless, ljut contains enormous quantities of spherical or stellate calcareous 
1 Die Synascidien der Bucht von Eovigao, p. 31, Wien, 1883. 
