HYDROZOA. 101 



cZcie" from earlier observers. Their true nature 

 was first demonstrated by E. Leuckart, who several 

 times witnessed the separation of these bodies 

 from a well-known species of Ahyla. Groups 

 of organs became detached from the coeno- 

 sarc, each group consisting of a hydrophyllium, 

 polypites, tentacles, and gonophores, with a small 

 portion of the coenosarc itself. More frequently, 

 however, the actual detachment of the Diphyozooid 

 has not yet been observed, so that the precise origin 

 of many still presents a subject for inquiry. Pend- 

 ing further investigation, it seems right to designate 

 such forms by provisional generic and specific 

 names, of which not a few have already been con- 

 ferred. 



6. Order 5 : Physopliorldae. — The Fliyso- 

 phoiHdce differ much more among themselves 

 than do the members of the order just mentioned. 

 All, however, agree in having the proximal end of 

 the coenosarc modified to form the pneumatophore, 

 or float, which presents so characteristic a feature 

 in the physiognomy of these animals. The cavity of 

 this pneumatophore is a simple enlargement of that 

 of the coenosarc, the walls of both being directly 

 continuous. To the apex of the cavity is attached 

 a firm, elastic, apparently chitinous sac, known as 

 the ' pneumatocyst,' containing a greater or less 

 proportion of air. A layer of endoderm, reflected 

 from the pneumatophore, invests the whole outer 

 surface of its contained pneumatocyst, which is thus 

 completely cut off from the somatic cavity below. 

 The lower extremity of the pneumatocyst is usually, 

 if not always, entire. Its apex, though most fre- 

 quently closed, is open in Physalla and RMzophysa, 



H 3 



