MOLLUSCA. 215 
as P. elegans of Lesueur; and Pyrosomata paniculata, 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 
branchiz, 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 branchie 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 branchiz 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- 
