SALPIDAE 10 1 



Sub-Order 2. Hemimyaria. 



Free-swimming pelagic forms which exhibit alternation of 

 generations in their life-history, and in the sexual condition form 

 colonies. The body is more or less fusiform, with the long axis 

 antero-posterior, and the branchial and atrial apertures nearly 

 terminal and opposite. The test is well developed but trans- 

 parent. The musculature of the body-wall is in the form of 

 a series of transversely -running bands which do not usually form 

 complete independent rings as in the Cyclomyaria. These 

 partially-encircling muscles in the Salpidae (see Fig. 61, ni.h) 

 are probably to be regarded as modified branchial and atrial 

 sphincters which have spread over the intervening body. The 

 branchial and peribranchial (cloacal) cavities form a continuous 

 space in the interior of the body, opening externally at the 

 ends by the branchial and atrial apertures, and traversed 

 obliquely from the dorsal and anterior to the ventral and 

 posterior end by a long narrow vascular ciliated band, which 

 represents the dorsal lamina, the dorsal blood- sinus, and the 

 neighbouring parts of the dorsal edge of the branchial sac of an 

 ordinary Ascidian. The alimentary canal is placed ventrally. 

 It may either be stretched out so as to extend for some distance 

 anteriorly, or, as is more usual, be concentrated to form along 

 with the testis a rounded opaque mass near the posterior 

 end of the body, known as the visceral mass or " nucleus." 

 The embryonic development is direct, no tailed larva being 

 formed. The embryo is united to the parent for a time by 

 a " placenta." 



This sub -order contains, in addition to its typical members, 

 the Salpidae, another still somewhat problematical family the 

 OCTACNEMIDAE, including a single very remarkable deep-water 

 genus (Octacnemus), which in some respects does not conform 

 with the characters given above, and exhibits a certain amount 

 of affinity with the primitive fixed forms from which Salpidae 

 have been derived. 



Occurrence and Reproduction. — The family Salpidae ^ in- 

 cludes the single genus Salpa, Porskai, which, however, may be 



1 The most useful works on the Salpidae are Traustedt, Vid. Selsk. SIcr. ii. 8, 

 1885, Copenhagen; and Brooks's "The genus Salpa," Johns Sophins Biolog. 

 Memoirs, ii. 1893. 



