EEPORT ON THE TIJHIOATA. 
255 
spicules, whicli give to most of the Didemnidse their characteristic appearance. Their 
presence renders the test hard, brittle, and opaque, and gives it in most cases a gleaming 
white appearance. Both by sight and by touch, a specimen of the Didemnidse 
with calcareous spicules in the test may be readily diagnosed without further examina- 
tion, while a lens or low power objective places the matter beyond all doubt by showing 
the numerous white stellate bodies imbedded in the test. In some few species, however, 
of the fa mil y {e.g., Didemniim tortuosiim, von Drasche), no spicules have been found. 
Whether this is permanently the case in these species, or is only at a certain time or in 
a certain condition of the colony, is not yet known. It is quite an exceptional circum- 
stance amongst Didemnidse. 
Another very important characteristic of the family is found in the condition of the 
reproductive organs. The male system consists of a single very large testis of ovoid 
form placed on the right side of the intestinal loop, and having a long conspicuous 
vas deferens, the lower part of which commences its course by coiling spirally around 
the upper part of the testis for a number of turns. It then runs forward in the 
usual manner to join the rectum, along which it courses to the peribranchial cavity. 
The ovary consists merely of a few ova which are placed alongside tiie vas deferens. 
The mature ova are of very large size, and they frequently make their way into the 
common test surrounded by a covering of ectoderm, which they push out before them 
from the body-wall of the parent Ascidiozooid. How their escape from the common 
test to the exterior of the colony is effected is not yet knovm. 
The alimentary canal is comparatively simple, and is of much the same relative 
size in all members of the family. The stomach is large, and is usually of more or less 
globular or ovate form. Its wall is almost invariably smooth, witii no folds, ridges, or 
other thickenings. In a few cases, however {e.g., LcptocUnum speciosum), slight ridges, 
formed of thickened epithelium, project into the interior, but they are not visible on the 
outside. In some cases the intestine may be divided into regions of different calibre 
{Eucodium hospitiolum, Savigny), and sometimes the margin of the anus is provided 
with a pair of curiously curled horns or projections (see von Drasche, Die Synascidien, 
&c., p. 30, and Taf. xi. fig. 34). 
The process of gemmation in the Didemnidse is very remarkable. Each new Ascidio- 
zooid in the colony arises as two buds which form respectively its thoracic and 
abdominal regions. The first of these develops from the region of the parent where the 
thorax and the abdomen join, while the second is formed a little farther back, on 
the abdomen. The two buds eventually join to form the body of the new Ascidiozooid. 
This method of budding was termed pyloric by Giard.^ It has since been more fully 
investigated by Della Yalle.^ Embryonic budding, or blastogenesis, is not seen so 
1 Recherches snr les Synasciclies, Archives de Zool. ex^dr., &c., t. i. p. 576, 1872. 
^ Nuove Contribuzioni, &c., p. 48, 1881. 
