MOLLUSCA. 517 



ced between the two branchia 1 , the latter directed towards 

 the anal orifice. The heart is contiguous to the stomach, 

 at the bottom of what may be termed the branchial sac, and 

 is enveloped in a membranaceous pericardium. The bran- 

 chiae are double, not incorporated with the walls of the sac, 

 but with two folds of unequal length. The largest is free 

 in the middle, fixed at each extremity, and opposed to 

 the dorsal groove, and traverse the cavity obliquely. The 

 other extends from the base of the first to the extremity of 

 the dorsal groove. The surface of the branchiae consists of 

 transverse vessels in a single range in some species, and a 

 double range in others. 



When young, many individuals often adhere, and form 

 chains and circles. But the fully grown individuals are al- 

 ways detached and single. 



This subdivision comprehends the species of the genus 

 Salpa ; they are exceedingly numerous, and appear to be- 

 long to many different genera. M. CUVIER has given in- 

 dications of some of these, chiefly derived from the shape. 

 A few are furnished with an elevated crest or fin, as the 

 Thalia of BROWN ; a few have both extremities rounded 

 or truncated, as Salpa octofera of CUVIER ) others have 

 one extremity produced, as Holothuria zonariaof GMELIN ; 

 and even both extremities produced, as Salpa maxima of 

 FORSKAEL. The Sulpa moniliformis, so common in the 

 Hebrides, and first recorded as a native by Dr MACCUL- 

 LOCH, in his valuable Description of the Western Isles, 

 Vol. ii. p. 188, and imperfectly figured in its young state, 

 at Tab. xxix. fig. 2., appears to be closely allied to the S. 

 maxima of FORSKAEL, and but very remotely with the S. 

 polycratica and confederata with which it is compared. 



This observer states, that " It cannot bear to be con- 

 fined in a limited portion of water, as it died even in a 

 ship^s bucket in less than half an hour. With us, in simi- 



