ON THE AFFINITIES OF THE FAMILY OF THE MEDUSA 29 



indebted), of reading M. Dujardin's " Mdmoires sur le Developpement 

 des Medusas et des Polypes Hydraires," contained in the Annales des 

 Sciences Naturelles for November 1845. This author has, as it appears 

 to me, been misled by the great analogy between the structure of a 

 Medusa and that of the generative organ of a Coryniform Polype, into 

 taking the detached organ of the Polype for a real Medusa. He does 

 not hesitate to say that the Claviform Polypes are " only a first stage 

 of development of the Acalepha;." He hints that each clavate Polype 

 has its corresponding Acalephe, and he does not hesitate to give the 

 latter distinct names as independent genera {Stiienyo, Cladoneind). 



Here, as in many other instances, the study of the Diphydse throws 

 light upon the matter. The detached free-swimming testis or ovary 

 of a species oi Sphenia has just as much claim to a distinct generic 

 name as has Sthenyo or Cladonema, and yet in what respect does this 

 differ from the persistent ovary of Etidoxia, which surely is an organ, 

 and nothing but an organ ? 



Would it not be as reasonable to give a distinct name to Needham's 

 sperm-sacs because they exhibit certain independent motions external 

 to the body of the Cephalopod ? 



The point is of consequence, because it is anything but desirable 

 that true polypes with medusifon?i generative organs should be con- 

 founded with the Polypifonn larvce of true Medusa. 



DESCRIPTION OF THE PLATES. 



*^* In all the sectional diagrams the letters have the same meaning, viz. m. Stomach. 

 11. Common cavity, u. Canals, p. Generative organ. (]. Natatorial organ, t. Tentacle. 

 II. Marginal vesicle, jt. Outer membrane, x". Bract, x". Valvular membrane. 



Pl. XXXVII. [Plate 2.1 



Thaitmantias ? 



Fig. I. Disc seen from above. 



Fig. I a. Imaginary vertical section. 



Fig. 2. Opening of the stomach into the canals seen from above. 



Fig. 3. Marginal tentacles. 



Fig. 4. Young generative organ. 



Rlesonema ? 



Fig. g. Lateral view of the animal. 



Fig. 5 a. Vertical section. 



Fig. 6. View of a segment of the disc ; under surface. 



a. Buccal tentacles. 



b. Canals. 



L. Marginal membrane (20). 



