The branchial sac of the Botryllidee is very similar to that of the Clavellinidx. The bran chial spiracles, or intervascular spaces, are variable in number, and the crest or fold cor responding to the anterior border of the branchial sinus has no membranous languet ; but in Diazona, Synoicunt, and Po/ye/intim it bears a row of minute tentacular filaments. The branchial sae of Si gillina has 4 large, salient, transverse vessels on each side, united by 15 or 16 smaller longitudinal vessels ; and in Aplidium there are 10 to 12 trans verse branchial vessels. Polyclinunz has 14 transverse branchial vessels anastomosing with 15 to 18 finer longitudinal vessels. In Ama rouciunz there are 10 to 12 rows of stigmata, and in Polyclinum 13 rows. Didemnunz and Leptoclinum have 5 rows. In Botrylloides the respiratory sac presents 10 vertical rows of spiracles, parcelled into threes by vertical folds. There are 9 of these chink-like open ings on each lateral rank, and at each of their four angles is a little tubercle. In Botryllus the respiratory sac lies almost horizontally, and has on each side 9 transverse rows of stigmata, grouped into threes by the longi tudinal folds. The angles of the branchial net-work are marked with papillae in Distoma and Diazona.
Instead of papillae on the spaces between the branchial meshes, Leptoclinum Listeri has a thin ledge between each row of spiracles ; and in front there are three tapering moveable prominences, one connected with each ledge, either stretched forward horizontally into the cavity, or bent downwards with a spiral curve. These laminae, observes Mr. Lister, seemed to suspend a generally invisible vertical mem brane, and to assist in giving the food its direction towards the stomach ; for it moved horizontally along the sides of the cavity, as in Perophora, and when it reached the front, took a spiral motion downwards. When the branchial sac of the Leptoclinum contracted forcibly to reject what had been stopped by the tentacles, or found unfit for food, the oral orifice, instead of projecting, was then drawn down below the level of the external test, and depressed it, the cilia being also closely stretched across the openings of the spiracles. When the cilia were thus stopped in their action, they were seen to be very numerous, and being in close contiguity one with another, the neighbours by their sides, and the oppo sites by their ends, appeared almost as a con tinuous membrane.
The superior abdomen of the Polyclina contains the digestive apparatus, and the post abdomen the organs of generation and the heart. The intestines of the Botrylliche are always subject to one or more foldings, and their several portions are frequently distin guished by different tints of colour.
The superior abdomen in Anzarozzcium is short and rounded, and slightly separated from the thorax. The oesophagus is very short ; the stomach in A. proliferunz is marked ex
ternally with a series of vertical folds, the edges of which, seen under the microscope, are furnished with secretory follicles. In A. argus, the exterior of the stomach is coated with a tessellated series of irregularly hexa gonal, gland-like, bodies, having the appear ance of a number of minute honeycomb com partments, around each of which are radiating lamellae or folds. In Sidnyunz the stomach is also surrounded with glands ; and in this genus the intestine is spirally folded. In .Diazona the stomach is striated externally ; its inner surface is provided with numerous salient folioles ; and the pyloric entrance is guarded with an annular fold or valvule. The intestine in its first part is simply membran ous, but afterwards it is furnished with irre gularly disposed glands, which have the form of cweal tubes. The stomach of Sigillina bears some slight ridges on its inner surface, the strongest of which correspond to external furrows. In Aplidiunz the stomach is some what oblong and truncate, and is divided by deep plications into three longitudinal cavities, or rather into five, the lateral ones being sub divided. The intestine is bent sometimes forward and sometimes backward ; in the latter case it passes upward obliquely on the right side of the stomach ; the rectum is sometimes spirally arranged, and sometimes straight. In Polyclinunz also the intestine is spirally folded, obliquely traversing the left side towards the anal orifice. In A. proli ferunz and A. argils the rectum ascends to the middle of the cloaca. The post-abdo men is elongated, narrow, and tapering, divided from the digestive cavity by a marked constriction. The ovary occupies nearly the whole length of this cavity (fig. 782. p), and is partially surrounded by the glandular mass of the testicle, from which latter an undulatory vas deferens runs upwards to end in the cloaca near the termination of the rectum. The ova are whitish, yellow, or brown, and on escaping from the ovary appear to pass to the cloaca by the vacant space between the intestines and the interior of the mantle. The heart is a tubular vesicle, bent on itself; and situated at the inferior apex of the ovary.
In the Didemnians the digestive, generative, and circulatory organs are grouped together in the pedunculated abdomen, and offer no thing remarkable. The ovary is situated by the side of the intestine, and protrudes down wards only when full of eggs, which in this case have, when fully developed, a large size, compared with that of the animals. The mantle of the Didemnum, as in many other Botryllidce, is frequently produced in the ab dominal region into stolon-like processes, which traverse the common test. These swell at their extremities into reproductive buds, from whence arise new animals. Of these proli ferous tubes we shall have to speak hereafter.