156 THE OCEAN WORLD. 



This beautiful Porpita was discovered by Lesson on die Peruvian 

 coast, where it occurred in swarms closely packed on the surface of 

 the sea. " Its manner of life," says Lesson, "is perfecdy analogous 

 to that of the Velella. Their locomotion on the sea is purely 

 passive, at least in appearance. Their disc laid fiat on the surface 

 upon the water-line, leaves them to float freely and in a horizontal 

 direction, the irritable arms hanging all round them." 



MeDU-SIDtE. 



We here include in this family also the family Liicernan'adce. 

 The true Medusidse were termed by E. Forbes, Cymnophthalmia, and 

 the term Steganophthalmia was applied to a large section of the Lnccr- 

 variadcv. In the first division, according to Professor J. R. Greene, 

 the umbrella-shaped organ is to be regarded as a nectocalyx, the size 

 and shape of which, in relation to the polypite with which it is con- 

 nected may also vary very considerably. The veil which surrounds 

 the open margin of the nectosac appears never to be absent. Four 

 longitudinal canals are sometimes present. From the margin depend 

 tentacles, and around the margin are found the vesicles or pigment 

 spots, which are supposed to be eye-spots, -and being covered by 

 a prolongation of the nectocalyx. In some genera the tentacles are 

 sdfif, and not contractile, as is common in most of the genera. The 

 reproductive organs are of the simplest kind. At a time when the 

 free gonophores of the Hydrozoa had not been perfectly studied, it 

 was the custom to regard these bodies as quite independent organisms, 

 and they were arranged under genera and species. At last the 

 singular resemblance borne by such forms to the Mediisidcz attracted 

 attention, and it was soon found that many of the Medusidcz were not 

 true individual organisms, but merely the reproductive buds of various 

 Hydrozoa, and the conclusion was too hastily come to that the whole 

 group of MedusidcX ought to be abolished. The researches of J. and 

 F. Muller, Gegenbaur, and the lamented Claparade, have indicated 

 the probable existence of a group of medusid forms, which appear to 

 be the immediate results of true generative acts, and not of gemma- 

 tion or fission. It appears safer, in the present state of our knowledge, 

 to conclude — i. That several of the organisms formerly described as 

 Medicsida: are the free gonophores of other orders of Hydrozoa. 2. 

 That the homology of these free gonophores with these simple 

 expansions of the body-wall which in Hydra and some other genera 

 are known to be reproductive organs by their contents alone, is proved 

 alike by the existence of numerous transitional forms, and an appeal 



