22 
THE VOYAGE OF H.M.S. CHALLENGER. 
in tlie Botryllidse (fig. 4, C., and PL III. fig. 17), wliere the branchial sac is relatively 
much larger than in other forms, and the oesophagus runs ventrally to open into the 
large stomach, which is inclined ventrally and a little anteriorly. The intestine after 
leaving the stomach runs anteriorly and then dorsally and then turns anteriorly again 
to become the short rectum. The second series of modifications consists in the twisting 
of the intestinal loop so that the stomach comes to lie upon the dorsal in place of the 
ventral edge of the intestine (see fig. 4, B.). This condition is seen in various degrees 
of perfection in some of the Distomidse and Didemnidse. 
The oesophagus is usually narrow, but may be of considerable length (PI. IV. fig. 7). 
The stomach is thick- walled and large. Sometimes its surface is smooth, but in many 
cases it has conspicuous longitudinal folds which project as ridges into the interior 
(PI. XXX. figs. 6, 7); occasionally these folds are represented by short detached cseca 
(PI. XXV. fig. 6). The intestine may be a simple tube, or may be divided into 
several regions distinguished from one another by their calibre and the thickness of 
their walls (PI. XLI. fig. 2). The rectum is always thin-walled, and is sometimes 
very vdde. It may vary in calibre at different points, and sometimes it is folded. The 
anus may have a large margin, either plain or lobed. In some cases it opens near the 
posterior and in others near the anterior end of the peribranchial chamber. 
Many of the Ascidise Compositse have a system of tubules branching over the wall 
of the intestine, which clearly corresponds to the digestive gland of a similar nature 
found in many Simple Ascidians.^ The duet from this system runs across from the 
intestine and opens into the stomach. A short caecum may be present near the posterior 
end of the stomach at the point where the duct opens (see PI. III. fig. 21). The exact 
function of this system is still unknown. 
Heart, Blood, &c. 
The heart varies in position in the different families. In the Botryllidse it is placed 
close to the stomach alongside the branchial sac. In the Distomidae, &c., it is placed 
alongside the intestinal loop on the right side, and runs antero-posteriorly. In the 
Polyclinidae the heart is placed at the posterior end of the post-abdomen behind the 
reproductive organs (see fig. 3, p. 16, A.), and is bent so as to form a loop concave 
anteriorly. The structure of the heart and of the blood is the same as in the case of the 
Simple Ascidians. A large number of the blood- corpuscles may in some cases be 
pigmented. These are usually opaque white or red or purple. 
The so-called vascular appendages, which form prolongations from the bodies of the 
1 See Part I. of this Eeport, Introduction, p. 49. Roule (Recherches sur les Ascidies Simples des Cotes de Provence) 
states that this is a system of blood sinuses ! 1 am convinced that in some of the species in which I have investigated it 
the main duct opens into the alimentary canaL 
