516 PHILOSOPHY OF ZOOLOGY. 



abdominal cavity. The E. hospitiolum of Sav. Tab. iv. 

 f. 4, is the only known species. 



2d Tribe. 



Body free and moving about in the water. 



75. Pyrosona. 



The body is gelatinous, in the form of a lengthened bag 

 open at the widest end. The individuals are arranged per- 

 pendicularly to the axis of the central cavity, super-imposed 

 on one another. The branchial orifice is external, without 

 rays, and with an appendage over its upper margin. The 

 anal orifice is opposite, and terminates in the central cavity. 

 Branchial sac destitute of folds, with a membranaceous ring 

 at the entry. The branchiae are disjoined. The abdomen 

 is inferior to the branchiae, and not separated by any con- 

 traction. Liver distinct, globular, and retained in a fold 

 of the intestine. Ovarium double, opposite, and situate at 

 the upper extremity of the branchial cavity. 



M. Savigny divides the species into Pyrosomata verti- 

 cilla, having the individuals arranged in regular prominent 

 rings, as P. elegansof Lesueur; and Pyrosomata panicu- 

 lata, having the individuals forming irregular circles une- 

 qually prominent, as P. giganteum and atlanticum. 



2d Subdivision. 



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 accelerate the entrance of the water into the cavi- 

 ty. The inner tunic is strengthened by numerous trans- 

 verse muscular bands, which, by contracting, diminish the 

 diameter of the cavity, and eject the water from the anal 

 orifice, thereby propelling the body through the water 

 The digestive orrans are situate at the inner end of the ca- 

 vity. The mouth and rectum are simple ; the former ])la- 



