SEA-SQUIRTS. 
567 
moseleyi ) the stem is much shorter, and there is only a single plate, situated on 
the dorsal side. Of very large dimensions, these deep-sea ascidians are decidedly 
the most beautiful members of the class, and present some resemblance to the 
glass-sponges. A totally different type of structure is presented by the last family 
(Clavelinidce) of the suborder, in which the body of each individual is attached 
by its posterior end, and usually by means of a stalk, to a creeping basal stolon, 
or common mass, from which young individuals are produced by budding. The 
outer tunic, which is usually thin and transparent, is in most cases gelatinous, 
although occasionally cartilaginous; and its circular apertures are but seldom 
distinctly lobed. Folds are wanting in the branchial chamber, but longitudinal 
bars may be present, although these lack the papillae found in the preceding- 
family; and the gill-slits are straight. The tentacles resemble those of the last 
family in their simple, thread-like form; but the digestive tract is usually 
extended behind the branchial chamber to form an abdomen. In addition to 
the ordinary sexual reproduction, colonies may be formed by budding from the 
common stolon. Ten genera are included in the family, from among which the 
typical Clavdina is selected for illustration. Here the bod}' is elongated and 
club-shaped, but with no peduncle beyond the abdomen, and is attached to a 
delicate, branched, creeping stolon, from which arise 
the buds. The thin outer tunic is gelatinous or 
cartilaginous, with its circular apertures devoid of 
lobes. The inner tunic is likewise thin, with its 
muscles mainly longitudinal; and the intestinal tract 
is extended to form a well-marked abdomen. In its 
restricted sense, the genus includes only half a dozen 
small species from North-Western Europe and the 
Mediterranean; the one here figured ( C. lepadiformis) 
being characterised by the yellow or brown lines on 
the region known as the thorax. 
The second suborder of the typical sea-squirts— 
Ascidke Compositas—includes fixed forms which repro¬ 
duce by buds so as to constitute colonies in which the 
individuals are buried in a common investing mass, and thus possess no separate 
tunics. The group includes seven families; and Professor Herdman remarks that 
as many of these have originated independently from simple forms, the whole 
assemblage is to a certain extent an artificial one. In the first family ( Botryllidce ) 
the colonies, as shown in the illustration on p. 5G8, usually form thin incrusta¬ 
tions on seaweeds or stones, although they occasionally take the shape of thick 
fleshy masses ; the individuals being arranged so as to form either circles or ellipses, 
or in branching lines. The common apertures of discharge are distinct, and usually 
furnished with lobes; the individual units are short, and show no division of 
the body into regions; and the outer tunic, which is usually soft, is traversed 
by numerous vessels with large terminal knobs. Internal longitudinal bars are 
present in the large and well-developed branchial chamber, in which the gill-slits 
are numerous; and the simple tentacles do not exceed sixteen in number. Budding 
may take place either from the sides of the units constituting the colony or from 
A CREEPING ASCIDIAN, Clavdina 
(uat. size). 
