THALIACEA. 45 



more or less complete circular bands, by the contraction of which 

 locomotion is effected. The pharynx has either two large, or 

 many small apertures leading into a single atrial cavity, which 

 opens to the exterior by the atrial aperture. The anus opens 

 into the atrial cavity. Alternation of sexual and asexual genera- 

 tions (metagenesis) occurs in the life-history and may be com- 

 plicated by polymorphism (Doliolidae). They are only occa- 

 sional visitors to British shores. A few species of Salpa and of 

 Doliolum have been taken in the seas around the Hebrides. 

 They are divided into two groups, the Cyclomyaria and the 

 Hemimyaria, which seem to be sufficiently distinct to merit the 

 rank of sub -orders. 



Sub-order 1. Hemimyaria (Salpida). 



Thaliacea in which the pharynx is reduced to its median dorsal 

 and ventral (endostylar) walls, the lateral walls being absent. The 

 asexual form produces chains of sexual individuals, which give rise 

 again to the asexual form. The muscular rings are usually incom- 

 plete ventrally and a tailed larva is not developed. 



The salps are transparent pelagic organisms, coloured and 

 opaque at one spot the nucleus, where the digestive organs and 

 heart are placed. The test is soft and gelatinous and the mantle or 

 body- wall closely adheres to its inner surface. The body is some- 

 what elongated, with the mouth in front and the atrial aperture 

 behind and slightly dorsal. The muscular tissue (Fig. 38) of 

 the body-wall is arranged in hoop-like bands usually six to nine 

 (there may be as many as twenty and as few as four) which are 

 continuous dorsally, but usually not ventrally, except behind 

 where they constitute the atrial sphincter. At the mouth they 

 are modified in a different way, being prolonged forward into the 

 lips. All these muscles are transversely striated. JDorsally 

 some of them frequently join or approach one another. It is by 

 the contractions of these bands with the mouth closed that water 

 is expelled from the atrial cavity, causing the movement of the 

 animal in a forward direction. The mouth is not lobed, but is 

 bounded by mobile upper and lower lips. It leads into the pre- 

 branchial part of the pharynx, which contains but a single ten- 

 taclethe so-called languet on its dorsal side (Fig. 40, 11) 



