MOLLITSCA. 215 



as P. elegans of Lesueur; and Pyrosomata paniculate, hav- 

 ing the individuals forming irregular circles unequally pro- 

 minent, as P. giganteum and Atlanticum. 



2. Subdivision. Monochtonida. 



Inner tunic adhering throughout to the external one. 



The body is gelatinous, transparent, and simple. The 

 branchial cavity is open at both ends, communicating freely 

 with the anus. The branchial orifice is in the form of a 

 transverse slit, with one edge in the form of a valve, to ac- 

 celerate the entrance of the water into the cavity. The 

 inner tunic is strengthened by numerous transverse muscu- 

 lar bands, which, by contracting, diminish the diameter of 

 the cavity, and eject the water from the anal orifice* there- 

 by propelling the body through the water. The digestive 

 organs are situate at the inner end of the cavity. The mouth 

 and rectum are simple ; the former placed between the two 

 branchiae, 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 branchiae 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 

 traverses 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- 



