PERIGONIMUS. 321 



Development of Gonosome. — April and May. 



Habitat. — Ou empty shells of Balani, &c., from the bottom of the sea. 



Locality. — Off the town of Reikcwig, in Iceland, Professor Steenstrup. 



I take for granted that Steenstrup is correct in regarding as the mature form of the plano- 

 blast of his Corj/ne FritUlaria a mcdus;i which he found swimming at large in the open sea, in 

 the neighbourhood of the spot which aflbrdcd him his specimens of the trophosome, and both 

 the generic and specific descriptions given above are based on this supposition. We cannot, 

 however, shut our eyes to the fact that in this identification we are dealing with a mere inference, 

 and that the free-swimming medusa has not been traced back by direct observation to the 

 tropliosome. 



Steenstrup describes a lobcd body as existing at the common base of the two marginal 

 tentacles in his free medusa. He regards this as the generative organ of the medusa, a view 

 which, however it may have been justified at the time when he wrote, will not be shared in by 

 any one at present. There can, I think, be little doubt that the body in question is a cluster of 

 young medusa-buds originating in a situation in which this form of gemmation is by no means 

 rare among the Hydroiba. 



PERIGONIMUS, Sars. 



Name. — From -foJ, around, and yon/uoc, productive ; so named from the disposition of the 

 gonophores round the stem and Ijranches of the trophosome. 



Atractylis (in part), — Stretliill IVriylit. 



TROPHOSOME. — Htdeophyton consisting of a brandling or simple htbko- 

 CAULTJS rooted by a filiform nxmiOEmzA. Hydrantiis fusiform, with a conical 

 liypostome. 



GONOSOME. — Planoblasts developed from the liydrophytou. Umbrella, at the 

 time of liberation, deep bell-shaped, with the oral extremity of the manubrium either 

 simple or more or less deeply lobed ; marginal tentacles either two or four, not in 

 clusters, and with bulbous bases, which are not furnislied with distinct ocelli. 



The genus Perigonimus was founded by Sars' for a hydroid which he obtained on the 

 Norwegian coast. The position of its medusae, scattered over the hydrocaulus, appeared to him to 

 offer a feature so peculiar as to entitle this character to be assumed as one of the chief grounds 

 for the establishment of a new genus, while it further suggested to him the generic name. 



1 'Fauna lit. Norv.,' Irste Liefer., p. 8. 



