Botkyllid^. MOLLUSCA. Distomus. 
384 
jections. These are Compound Ascidians. A tangle 
or broad-leaved fucus, torn from its rocky bed, or 
gathered on the sands where the waves have cast it 
after storms, will show us similar bodies, mostly those 
star-figured, investing its stalks, winding among the 
intricaeies of its roots, or clothing with a glairy coat the 
expanse of its foliated extremities. If we keep some of 
these bodies in a vessel of sea-water we find they lie 
as apathetic as sponges, giving few symptoms of vitality 
beyond the slight pouting out of tube-like membranes 
around apertures which become visible on their sur- 
faces, though a closer and microscopic examination will 
show us currents in active motion in the water around 
these apertures, streams rejected and whirlpools rush- 
ing in; indicating that however torpid the creature may 
externally appear, all the machinery of life, the respira- 
tory wheels and circulatory pumps, are hard at work in 
its inmost recesses. In the course of our examination, 
especially if we cut up the mass, we find that it is not 
a single animal which lies before us, but a common- 
wealth of beings bound together by common and social 
ties. . . . Each member of the commonwealth has 
its own peculiar duties, but shares also in operations 
which relate to the interest and well-being of the mass. 
Anatomical investigation shows us the details of these 
curious structures and arrangements beautiful as wise. 
Indeed few bodies among the lower forms of animal 
life exhibit such exquisite and kaleidoscopic figures as 
those which we see displayed in the combinations of the 
Compound Ascidians.” — (Forbes.) The Compound 
Ascidians form rather a large family, comprising within 
it at least thirteen distinct genera. These may be 
arranged, however, in three groups or sub-families. 
Sub-family I. — BOTKYLLINA. 
In the animals of this group the individuals are 
united in systems around common excretory cavities 
or cloacce; and their bodies are not divided into a dis- 
tinct thorax and abdomen. 
Genus Botryllus. — This genus may be taken as 
the type of this group. The external tunic is gelatinous 
or cartilaginous, sessile, and incrusting. The animals 
are grouped in simple prominent stars, and the indi- 
viduals — from six to twenty in each system — lie hori- 
zontally, with the vent far from the branchial orifice, 
which is simple. The branchial orifices are arranged 
round a common cloaca. “The tests of the animals 
are fixed together, forming a common mass, in which 
the animals are imbedded in one or more groups or 
systems, but the individuals are not connected by any 
internal union.” — (Adavis.) There have been as 
many as ten distinct species described, six of which are 
found on the shores of Great Britain. They vary con- 
siderably in form and colour, some of them being 
purple, and others yellow, blue, grey, or green. They 
are fixed on stones or sea-weed, near low-water mark. 
I’he genus is represented by B. jjolycyclm (Plate 9, 
figs. 7 and 8). 
Sub-family IB— POLYCLININA. 
This group is distinguished from the preceding by 
the body being divided into three distinct portions, viz. 
— a thorax, with the branchial apparatus; a superior 
abdomen, with the digestive organs; and a post- 
abdomen, with the heart and reproductive organs. 
Genus Polyclynum. — This genus has a gelatinous 
or cartilaginous test, sessile or slightly pedunculated, 
and varying very much in form. The systems are 
numerous, convex, somewhat star-shaped, each with a 
central cloacal cavity. The individuals, varying from 
ten to an hundred and fifty, are placed at unequal 
distances from their common centre. The abdomen is 
much smaller than the thorax, and the post-abdomen is 
pedunculate. The branchial orifices are six-angled and 
six-rayed, and the anal aperture is prolonged hori- 
zontally and irregularly cut. Six species are known, 
one of which, P. constellatum, is %ured in Plate 9, 
figs. 12-14. 
Genus Synoicum. — This genus has the test semi- 
cartilaginous, cylindrical, and pedunculate, and the 
animals are either isolated or attached gregariously to 
similar cylindrical bodies. The systems are single, 
circular, terminal, and comprise six to nine animals. 
The branchial orifices are six rayed, and the anal 
aperture has the same number ; but they are unequal, 
the three largest forming the exterior margin of the 
central star. Only one species is known, S. turgens, 
figured in Plate 9, figs. 15—17. 
Genus Sigillina. — This genus has a solid, gelatin- 
ous test, in form of an elongated, erect cone. It is 
pedunculated, and occurs either isolated or gregar ious. 
Each cone consists of a single system of many indivi- 
duals disposed one above another in irregular circles. 
The branchial and anal orifices have each six rays; 
the abdomen is larger than the thorax ; and the post- 
abdomen is long and slender. The genus derives its 
name from a fancied resemblance of the common mass 
to a seal (sigilluml), and corrtains as yet only one species, 
S. australis, from the tropical seas, which is repre- 
sented in Plate 9, figs. 10 and 11 (magnified). 
Sub-family III. — DIDEMNINA5. 
This group embraces all the compound Ascidians 
whose body is distinctly divided into two parts, the 
thorax and abdomen. 
Genus Didemnium. — This genus has a coriaeeous 
test which is sessile and encrusting, and diversified in 
form and appearance. The systems are numerous, 
compressed, without central cavities, and the individuals 
are scattered over the common body without any appre- 
ciable order of arrangement. The abdomen is pedun- 
culate. The branchial orifice only is rayed. 
Genus Distomus. — This genus differs from the 
preceding in having both the branchial and anal orifices 
rayed. The test is semicartilaginous, sessile, and 
assumes various forms. The systems are numerous, 
usually circular, and the individuals are placed in one 
or two rows at unequal distances from a common cen- 
tre. The rays at the margin of both apertures are 
six in number. There are several species of this 
genus, one of which, D. ruber, is represented in Plate 
9, figs. 1-3. 
