26o THE BIOLOGY OF THE SEA-SHORE 



At Plymouth, Sycon coronatum^ Grantia compressa, and 

 Leucosolenia sp. were found to be annual by Orton (19 14). 

 This has been confirmed by Dendy for G. compressa. 

 According to Orton, there are two breeding seasons at 

 Plymouth, summer and late autumn, and there is evidence 

 that the same individuals may breed twice : once in late 

 summer and again in the summer following, after which 

 they die down. Growth continues through the winter and 

 temperature appears to be the main factor in reproduction. 



With regard to the growth rate, an exceptionally fine 

 specimen of Sycon coronatum attained a length of more than 

 28 cms. with an average width of about 2*2 cms. during a 

 period extending from July to the following May. Between 

 June and the following January a colony of Halichondria 

 panicea grew, on a flat surface, to cover an area of about 

 45 sq. cms., the thickness of the sponge being aboutthenormal. 



Hydrozoa. — The marine members of this class normally 

 occur in two forms : as a fixed asexual colony, the hydrosome 

 or hydroid, and a free-swimming sexual stage, the medusome 

 or medusoid. The actively-swimming medusoid forms an 

 efficient means of distributing the sex-cells, the dangers to 

 which such delicate organisms are exposed being reduced 

 by their liberation during the warm months of the year only, 

 at the season of calms. Hydroid and medusoid are not 

 always equally predominant in the life-history. In several 

 shore forms, e.g. the common Tuhularia indivisay the eggs 

 are produced on the parent stock within structures known as 

 sporosacs, which correspond to the medusoids but are 

 sessile. The eggs ripen within the sporosacs and eventually 

 the young are liberated as free-swimming embryos. The 

 sporosacs may be pendent and naked as in the species above 

 named, clustered at the bases of the tentacles, e.g. Clava 

 squamata, or contained in a special cup or gonotheca, c.^.some 

 Campanularians. Hickson (1906) gives the following table 

 to illustrate the relative importance of the different stages 

 in some common forms. The habitat in each case has been 

 added by ourselves. In these formulae the fertilised ovum 

 is represented by O, a conspicuous medusome stage by M, 



