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 

 (Clavelinidoj) 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: ami 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 Clavelina is selected for illustration. Here the body 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 (0. lepadiformis) 

 being characterised by the yellow or brown lines on 

 the region known as the thorax. 



The second suborder of the typical sea-squirts — 

 Ascidire Compositse — 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 firsi family (Botryllidce) 

 the colonies, as shown in the illustration on p. 568, usually form thin incrusta- 

 tions on seaweeds or stones, alt hough they occasionally take the shape of thick 

 fleshy masses; the individuals being arranged so as t>> form either circles or ellipses, 

 or in branching lines. The common apertures of discharge are distinct, ami usually 

 furnished with lobes: the individual units are short, and show do division of 

 the body into regions: and tin; 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 



3 



\ i 1:1:1 ri\<. i.S( ii>i an, < 'lavelina 

 (nat. size). 



