58 VERTEBRATE ZOOLOGY 



right as an atriopore. The salpian is therefore not twisted into a 

 U-shaped body but is secondarily somewhat straightened out, as 

 compared with an ascidian, since the atrium opens in the opposite 

 direction from the mouth. There is, however, evidence in the posi- 

 tion of the small, bent stomach and intestine that these now free- 

 swimming forms have been derived from fixed ancestors. 



Life Cycle of a Salpian (Doliolum). — An alternation of generations 

 exists between a sexual generation and a sexless generation, which 

 reproduces only by budding. The eggs produced by the sexual form 

 just described for Doliolum go through a typical early development 

 and produce tadpole larvae with large body and comparatively small 

 tail (Fig. 30, A). The whole animal is embedded in a thick coat of 

 jelly. These "tadpoles" undergo a metamorphosis by resorption of 

 the tail, and produce a type of individual much hke the sexual in- 

 viduals, called nurses (Fig. 30, B). On the ventral side near the heart 

 they have a short finger-like process which produces primary buds by 

 a process of constriction. These buds migrate over the surface of the 

 nurse by ameboid movement of the peripheral cells, and take up their 

 positions in three longitudinal rows on the cadophore, a dorsal process 

 of the body that comes off near the atrial end. The two lateral rows 

 are specialized as nutritive individuals and never leave the nurse. Of 

 those in the median row some become detached early and form a 

 second generation of foster-parents or nurses, while the rest grow to be 

 sexual individuals and ultimately separate one after the other from the 

 nurse. 



Colonial Salpians. — The genus Salpa is the only representative 

 of the Thaliacea that is colonial in the sexual condition; chains of 

 individuals that remain as colonies, adhering together by means of 

 their mantle walls, are released by the nurse. The development 

 from the egg to the nurse is direct, without a free-swimming tadpole 

 stage. In other respects they are quite like the solitary salpians. 



Obder III. Larvacea (Appendicularians) 



These are free-swimming pelagic forms with a much simplified 

 body region and a persistent tail like that of the larval ascidian or 

 salpian, supported by a well-developed notochord. The whole body 

 (Fig. 31) is usually inclosed in a voluminous gelatinous envelope or 

 test, called a "house," which is secreted by the mantle ectoderm just 

 as a tunicate secretes its wooden tunic. The "house" is^ however. 



