2944 THE SEA SQUIRTS OR ASCWIANS 



but soft and thin, although thickened in places to form plates. The internal 

 longitudinal bars usually found in the branchial chamber are wanting in this 

 genus; the gill slits are small and irregularly placed; and the viscera form a com- 

 pact irregular mass on the dorsal side of this chamber. In the species figured 

 (H. calycodes), which is from the North Pacific, the stem is of great length, and 

 the outer tunic thickened so as to form a number of nodules or plates; but in the 

 South Atlantic form (H. 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 pre- 

 sented by the last family (Clavelinidte} 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 pre- 

 ceding 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 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 Northwestern Europe and the 

 Mediterranean; the one here figured (C. lepadiformis} 

 being characterized by the yellow or brown lines on 

 the region known as the thorax. 



The second suborder of the typical sea squirts 

 A CREEPING ASCIDIAN, Clavehna. 



(Natural size.) Ascidiae 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 first family {Botryllidce} 

 the colonies as shown in the illustration on p. 2945, usually form thin incrustations 

 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 



