REPORT O^s THE TUNICATA. 
81 
The Ascidiozooid (PL V. fig. 13) may be di%dded iuto the body, the vascular 
appendage (r. ap), and the incubatory pouch (h. p.). The body, like that of all the 
Distomidge, consists of two regions, the thorax and the abdomen. Of these the thorax is 
rather the larger, and is the more anterior or superficial of the two. It is flattened 
laterally and is rudely quadi'angular in shape, the ends being anterior and posterior, the 
margins dorsal and ventral, and the sides right and left. It has the branchial and atrial 
apertures at its anterior extremity, is united to the abdomen ^posteriorly by a narrow 
neck, and has the incubatory pouch as a large diverticulum on its dorsal edge. The 
thorax consists of the branchial and atrial siphons, the branchial sac with all its contained 
organs, the tentacles, the nerve-ganglion and the neural gland, the upper part of the 
oesophagus, the terminal parts of the rectum and genital duct, and the atrium or peri- 
branchial space — the whole being surrounded by the mantle. 
The abdomen lies posterior to or deeper than the thorax, to which it is united by a 
narrow neck formed of the oesophagus, the rectum, the genital duct, and a covering of 
mantle. The abdomen is smaller than the thorax, is of an oval or ellipsoidal shape, and 
has the long vascular appendage attached to its posterior extremity. It contains the 
remaining organs of the body, namely, the stomach and intestine, the genital glands, 
and the heart. 
The vascular appendage starts from the posterior end of the abdomen on the right 
side, and runs, as will be more minutely described hereafter, inwards and then downwards 
through the colony. 
The incubatory pouch is a large spirally coiled diverticulum from the peribranchial 
space, vdth the dorsal part of which it communicates by a narrow duct. 
The mantle forms the outer wall of this irregularly shaped body, the parts of which 
have just been enumerated. It does not vary much in thickness ; the part covering the 
abdomen, however, is thinner than that on the thorax, which, excluding the siphons, is 
nearly equally strong all over; the neck of the incul>atory pouch is thicker than the rest 
of that sac, the farther end being rather weak. The parts forming the two siphons are 
the strongest of all (PI. VI. fig. 2), and the marginal lobes of the branchial aperture 
especially predominate (PL VI. fig. 4, hr.). 
Histologically the mantle is composed mainly of three elements — the connective 
tissue, the muscles, and the epithelium. The connective tissue is the ground-work, and 
is present in the form of a thin hyaline homogeneous membrane, containing many 
scattered cells ; it encloses and joins the bundles of muscle fibres, and is clothed on its 
inner surface by a layer of tessellated epithelium. The connective tissue cells are round, 
fusiform, and stellate in form, have granular protoplasm, and generally distinct nuclei 
which stain readily with carmine. In some parts of the mantle, as, for example, on the 
thin part of the incubatory pouch, many stellate cells are present, and the long processes 
of adjacent cells unite, forming an intricate network. On the narrow neck of the incu- 
(ZOOL. CHALL, EXP. — PART XXXVIII. 1885.) Pp H 
