Io8 TUNICATA SALPIANS 



CHAP. 



tail found in the larval condition of most Ascidians. The develop- 

 ment is direct ; and it may be said, then, that this young asexual 

 (solitary) Salpa differs from the corresponding form in the life- 

 history of Doliolujn (Fig. 60, A) in that its tail is no longer a 

 locomotory organ, but is represented by a nutritive mass, the 

 elaeoblast, while the body, in place of being free, is attached by its 

 ventral surface to a special organ of nutrition — the " placenta " 

 — in connexion with the blood-stream of the parent. 



This embryo sexually produced inside the body of an aggre- 

 gated form becomes a solitary Saljpa (such as Fig. 61, B), which 

 differs in appearance, structure, and habits from its parent, and 

 has no reproductive organs. After swimming for a time, how- 

 ever, it develops the ventral stolon on which buds form which are 

 eventually sexual Salpae. These are set free from the solitary 

 form in sets, still connected together, and they may swim about 

 together for a time as a chain of aggregated Salpae before separating 

 to become the adult sexual individuals (such as Fig. 61, A). 



Classification. — Salpa may be divided into the following sub- 

 genera : ^ — Cydoscdpa, Blainville, in which the alimentary canal is 

 ortho-enteric, and the " chain " consists of individuals united in a 

 circle ; lasis, Savigny, with several embryos formed at a time ; and 

 Pegea, Sav., Thalia, Blumenbach, and Salpa, Forsk§,l, all with 

 one embryo only, and differing from one another in the condition 

 of the " gill " and other details : all except Cydosalpa have the 

 alimentary canal caryo-euteric. Gydosalpa has three species, the 

 best known of which is G. jnnnata of the Mediterranean, a form 

 possessing light-producing organs like those of Pyrosoma, but 

 placed along the sides of the body. Salpa has four or five species, 

 one of which, S. runcinata-fusiformis (Fig. 61), has occasionally 

 been found in British seas; Thalia includes the species 2\ 

 deinocratica-mucronata, which has been sometimes obtained in 

 swarms in the Hebridean seas, or cast ashore on our southern or 

 western coasts ; Pegea has the species P. sc^itigera-confoederata ; 

 and lasts contains the remaining half-dozen species, the best 

 known of which is I. cordiformis-zonaria, the only other Salpian 

 which has been found in British seas. 



The family Octacnemidae includes the single remarkable 



For a more detailed account of these subdivisions of the Salpidae, and other 

 groups, see Herdman's "Revised Classification of Tuuicata," Journ. Linn. Soc, 

 Zool., xxiii. 1891, p. 558. 



