46 



EMBRYOLOGY 



same initial form. For if we assume that the alternation 

 of generations in Hydroids has arisen as a result of division 

 of labour, whereby the capabilities of sexual and non-sexual 

 reproduction have been distributed to different individuals 

 (Leuckart, 'No. 11), we must regard the different shapes of 

 these individuals as having been evolved from the same 

 fundamental form (Allman, No. 15 ; Glaus, No. 62 ; O. and R. 

 Hertwig, No. 8), the sessile individuals, which are repro- 

 duced exclusively by 

 budding, having under- 

 gone development more 

 in the vegetative direc- 

 tion, while the free- 

 swimming medusae, 

 which become sexually 

 mature, have allowed 

 the systems of organs 

 pertaining to the animal 

 functions to attain to 

 complete development. 

 Various circumstances ^ 

 indicate that in the ses- 

 sile form of the hydroid 

 polyp we have to do 

 with the primitive con- 

 dition, so that we may 

 characterize the hydro- 

 medusa as a metamor- 

 phosed hydroid polyp 

 which has acquired the 

 power of independent 

 locomotion. Then the 

 mouth of the medusa 

 would be homologous to 



1 As such is to be considered the fact that the sequence in budding 

 is from hydroid polyp to medusa, and never the reverse, further the total 

 absence of the production of organs, especially sensory organs, on the 

 exumbrellar side of the medusa-bell, which points to an antecedent 

 sessile condition ; this is of importance in comparison with the condition 

 in Ctenophores. 



Fis. 17.—^, diagram of a hyclrcid polyp; 

 B, of a craspedote medusa (after O. und R. 

 Hertwig, from Lawg's LeTirhuch). o, mouth ; 

 g, gastral cavity ; t, tentacle ; si, sustentative 

 lamella; gt, gelatinous mass between ecto- 

 derm and entoderm; rfc, radial canal; gl, 

 vascular lamella or cathammal plate; v, 

 velum ; rik, ring-canal. 



