II SALPIDAE TOW 
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 CycLtomyariA. These 
partially-encircling muscles in the Salpidae (see Fig. 61, m.d) 
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 SALPmpAE* in- 
cludes the single genus Salpa, Forskal, which, however, may be 
1 The most useful works on the Salpidae are Traustedt, Vid. Selsk. Shr. ii. 8, 
1885, Copenhagen; and Brooks's ‘‘The genus Salpa,” Johns Hopkins Biolog. 
Memoirs, ii. 1893. 
