REPORT ON THE THNICATA. 
17 
more “vessels” are contimied downvrards into tlie test (see fig. 2>,v.ap.). The Ascidio- 
zooid is of this form in the family Polyclinidse. In the group Didemniens there is no 
post-abdomen. The reproductive organs and the heart are placed in the abdomen along- 
side the intestine, so that the body consists of two divisions only (PL V. fig. 13), 
The Ascidiozooid is of this form in the follovdng families : — Distomidse, Didemnidse, and 
Diplosomidse. Finally, in Milne-Edwards’ Botrylliens, the body cannot be divided into 
regions. The alimentary canal, the reproductive organs, and the heart are all placed 
alongside the branchial sac or project very slightly beyond it (PI. II. fig. 7). The post- 
abdomen and abdomen seem to have been drawn upwards into the thorax. The families 
Botryllidse and Polystyelidse have Ascidiozooids of this form. 
In some cases the division into the above described regions is very distinct, the body 
being constricted between them so as to form two (Didemniens) or three (Polycliniens) 
masses united by narrow pedicles (PI. XXL fig. 5) ; but in other cases, although the 
regions are anatomically present, they are not apparent without dissection, the body 
being equally or nearly equally thick all the wmy down (PI. XXVI. fig. 4). 
The Mantle. 
The mantle has precisely the shape of the Ascidiozooid, of which it forms the 
covering inside the ectoderm. The latter separates it in all parts from the test, with 
which it therefore never comes into direct contact. The mantle is formed by connective 
tissue uniting and enclosing bundles of muscle-fibres, nerves, and blood-spaces. In its 
histology it resembles closely the mantle of the Ascidiae Simplices. It varies consider- 
ably in its thickness and in the condition of its musculature in the different species of 
Compound Ascidians. In some cases, both longitudinal and transverse (circular) muscle 
bands are present, and the transverse bands are the strongest and the most regularly placed. 
Figure 10 on Plate X. shows this condition of the mantle. In others they are placed with 
very great regularity, and the longitudinal bands are almost absent. In other Compound 
Ascidians, however {e.g., Colella ramulosa, PI. XV. fig. 17), the longitudinal muscle 
bands are more marked and more regular than the transverse ones, and, in some 
cases, the latter are almost absent while the longitudinal bands are then very powerful. 
In some few cases {Tylohranchion sp>eciosum, see PI. XXII. fig. 4) the muscle bands 
over the branchial sac are strong, and branch like those of a typical Ascidia; usually 
they are more delicate than in any of the Ascidiidae. The muscle fibres in the mantle 
are always unstriped. The sphincters surrounding the branchial and atrial siphons 
(where the latter opens independently) are usually well^^marked in Compound Ascidians. 
In some forms {e.g., Colella pedunculata, see p. 89) the median dorsal part of the 
mantle near the anterior end of the peribi;anchial cavity is prolonged to form a diverticulum 
which varies in size from a scarcely perceptible enlargement to a long spirally coiled 
(ZOOL. CHALL. EXP. PAET XXXVIII. — 1886.) Pp 3 
