116 



COMPARATIVE ANATOMY 



CHAP. 



from the stock, but remain connected with it as medusoid gonophores. 

 From the fertilised eggs of such Hydroids other Hydroids are produced. 

 On the other hand there are Hydro-Medusae in whose whole life cycle 

 no attached Hydroid stock is developed. From the fertilised egg of such 

 a Craspedote Medina another sexual Medusa is produced, often after a 

 series of metamorphoses. 



In the Discomedusce also a kind of alternation of generations occurs. 

 The fertilised egg may develop into a young attached Medusa, which 

 reproduces asexually by axial budding (strobilation, Fig. 81, p. 107) 



p - x^- 



FIG. 88. Nausithoe. pr, Perradii ; ir, interradii ; ar, adradii ; sr, subradii ; rl, marginal lobes ; 

 t, tentacles ; gf, gastral filaments ; m, circular muscle of the subumbrella ; sfc, sensory bodies 

 (rhopalia) ; g, sexual glands (gonades) ; in the middle the oral cross. 



or by lateral budding. The constricted young Medusae (ephyrse), 

 whose organisation, but for absence of gonades, is essentially the same 

 as that of Nausithoe (Fig. 88), undergo a more or less complicated 

 metamorphosis, till they again become adult sexually mature Medusce. 

 Here, however, the organism which multiplies asexually is really a 

 young stage of the sexually differentiated Medusa, not a sister as in 

 the Hydro-Meduscz. The young Scyphistoma does not need to multiply 

 asexually. It can detach itself from the stem and develop direct into 

 a Medusa. There are also very many free-swimming Scypho-Medusce 

 from whose fertilised eggs a new Medusa is produced again direct 

 without the intervention of an attached stage in which multiplication 

 is asexual. This direct development is usually accompanied by meta- 



