MUSEUM OF COMPARATIVE ZOOLOGY. 271 



able from the peduncle itself. There are four clusters of sexual bodies arising 

 from this enlargement of the peduncle. Each of these subordinate clusters is 

 attached by its own peduncle, and l)ears several buds in various stages of 

 growth. Of these buds the more proximal are generally the most developed, 

 and they have not yet taken on the form of a bell. The axis and peduncle of 

 each cluster is very contractile. The sexual cluster is a very prominent body 

 on the stem, swaying backward and forward with passing currents of water. 

 None of the single bells which form this cluster were observed to separate from 

 the axis, nor were their contents seen to be discharged. In none of the buds 

 could an egg be distinguished, and it is impossible to tell whether the animal 

 was male or female, so little is known of the difference between the sexes in 

 the genus Ehizophysa. " Mace-like " sexual organs, such as are found in 

 R. filiformis, probably develop into sexual glands similar to those which we 

 have described in R. gracilis* 



Many parasitic Crustacea were found infesting R. gracilis. They clung to 

 the float and polypites with such pertinacity that it was almost impossible to 

 tear them away without rupturing the Rhizophysa. Six of these parasites were 

 found upon a single Siphonophore. The same, or a closely related parasite,f is 

 also one of the greatest enemies of the genus Physophora. 



Athorybia formosa, sp. nov. 



Plate V. Figs. 3, 4. Plate VI. Figs. 7-14. 



A new species J of Athorybia was taken at the Tortugas. This remarkable 

 genus has never before been found in American waters, and very little is 

 known of the anatomy of the other species, A. rosacea, although it is very com- 

 mon in some parts of the Mediterranean. § 



Athorybia differs from other Physophoridce in the absence of a long axis and 

 nectocalyces, altkough possessing a well-developed float, and bracts or covering- 



* See Proc. Bost. Soc. Nat. Hist., XX., Note on Rhizophysa. The likeness of 

 the sexual organs of R. gracilis to the bundles of "medusa buds" iu Physalia and 

 Tubularia is very striking. Whether the medusoid buds always remain attached 

 in this Phj-sophore, as in Tubularia, is unknown. The resemblance between the 

 sexual organs of Physalia and R. gracilis is so close that the embryology of the two 

 genera must be very similar. 



t The genus and species of this Crustacean were not determined. 



\ Three species of Athorybia (sensu strictiori) are described by Eschscholtz, A. 

 helianfha, A. mclo, Q. & G., and A. rosacea. Since his time nothing has been added 

 to his descriptions of the two former. Practically what is known of the anatomy of 

 the genus we owe to the accounts of A. rosacea by Kolliker (Die Schwimmpolypen 

 Oder Siphonophoren von Messina, pp. 24-28, PI. VII.), Sars (Middelhavets Littoral 

 Fauna, Nyt. Magaz. f. Katur., X. pp. 6, 7), and Huxley {op. cit., pp. 86-89, PI. IX.). 

 A. formosa differs from their account of the anatomy of A. rosacea in the form of the 

 tentacular knob. 



§ Rare at Naples and Villa Franca. 



