64 



INVERTEBEATA 



CHAP. 



At the same time Gotte's results have thrown light on how 

 a medusa was developed out of a hydroid form. It used to be 

 held that a medusa was essentially a hydroid shortened in the 

 direction of the mouth-foot axis. This shortening, it was thought, 



had caused the oral and aboral walls of 

 the peripheral portions of the stomach 

 of the hydroid to adhere to one another, 

 and so to form a solid plate of endoderm, 

 the so-called endoderm lamella; thus 

 leaving the lumen only in the centre, in 

 the extreme outer edge (the circular 



canal), and in four radiating lines (the 

 radial canals). 



But if we follow Gotte we must 

 imagine a simpler process of evolution. 

 The bell of a medusa is, according to him, 

 merely a web connecting the basal parts 

 of the tentacles of a hydroid, and a 

 medusa is related to a hydroid as a duck's 

 foot is related to a hen's foot. 



We may suppose, then, that originally 

 the hydroid persons were separated from 

 the mother, and crept about, as still 

 happens in the case of the buds of Hydra, 

 and that these persons eventually de- 

 veloped genital organs ; but that a 

 differentiation in these buds took place, 

 so that some never separated, hut 

 remained permanently immature and 

 asexual, whilst those that did separate 

 developed a swimming web. In this way 

 the alternation of generations so char- 

 acteristic of Hydrozoa was developed. 



SIPHONOPHOEA 



Fig. 42. — Two longitudinal sec- 

 tions through the developing 

 gonophore of Clava, squainata. 

 (After Harm. ) 



A, young gonophore with rudi- 

 mentary umbreUa-cavity and unripe 

 ovum embedded in endoderm. B, fully 

 developed gonophore with ripe ovum, 

 ov, ovum ; w, umbrella-cavity. 



The Siphonophora are floating or 

 swimming Hydromedusae. The most 

 ingenious and plausible hypothesis as to their origin is that put 

 forward by Korschelt and Heider (1890), who regard as most 

 primitive those forms like Physalia and Velella, which float only, 

 and are without those engines of propulsion known as nectocalyces. 

 Korschelt and Heider (1890) suppose such forms to have been 

 derived from larvae of ordinary Hydromedusae, which have fixed 

 themselves to the surface film of the water. 



That this is a possible and even probable contingency will be 

 self-evident to any one who has watched young starfish walking 

 upside down on the surface film, like flies on the ceiling of a room, 



