British Protozoa and Zoophytes. 



127 



Perfect free Medusa of higher type ; 1 

 peduncle four-lipped or tentacled ; eight ( 

 generative sacs, one on each side of ^^i^ta. 



lateral canals ) ^ 



Medusa of 



Laomedea 



Medusa of highest type, with 4-6 lips and "1 Stomobra- 

 8-12 lateral canals, each canal carrying > chium octo- 

 a single generative sac J costatum. 



^ Peduncle and 

 generative 

 sacs differen- 

 tiated. Ali- 

 mentary po- 

 lyp taking 

 no part in 



. reproduction. 

 Generative 

 sacs on sepa- 

 rate canals; 

 <{ two canals 

 correspond- 

 ing to each 

 Up. 



Of the generation of Stomobrachium we know nothing. Cla- 

 parede has shown that gymnophthalmatous Medusae may pro- 

 duce Medusae without the intervention of the polypoid phase ; 

 but it is impossible to draw any line of distinction between a 

 Medusa and the raedusoid phase of the Hydroid polyp. Tubu- 

 laria indivisa produces its young as perfect polyps without the 

 intervention of the planuloid phase, Clava with the intervention 

 of that phase. In the life-history of the Hydroidse any phase — 

 planuloid, polypoid, or medusoid — may be absent. 



The perfect several-lipped Medusa appears to be a symmetrical 

 organism composed of eight or more elements, each element 

 corresponding to the half of a lip. Each of these elements is 

 composed of three subelements, the alimentary, reproductive, 

 and prehensile, any of which may be suppressed, or unite with 

 others of different value on the same element, or of the same 

 value belonging to neighbouring elements. Thus, in Sarsia 

 the peduncle appears to consist of a single alimentary subele- 

 ment, and the single reproductive element or generative sac 

 extends around and along the whole of it except the single 

 trumpet-shaped lip. This lip is occasionally placed on one side 

 and at some distance from the extremity of the peduncle, indi- 

 cating the asymmetrical character of the latter organ in this 

 genus. In Euphysa and Eleutheria the ovisacs coalesce, and 

 are placed within and at the base of the peduncle. Steenstrupia 

 and Saphenia furnish examples of the suppression of certain of 

 the marginal tentacles or prehensile subelements, and the exag- 

 geration of others. 



The Polyp of the Hydroid Zoophyte must also be considered 

 as composed of one or more elemental zooid. Thus we have the 

 zooid of a single element in the ' tentacular polyp ' of Hydrac- 

 tinia ; the zooid of two elements in the two-teutacled and two- 

 lipped Lar Sabellarum (Gosse) (PI. V. fig. 8), and in the minute 

 two-lipped and non-tcntacled polyp which occurs on the Anteu- 



