76 INVEETEBRATA CHAP. 



to leave practically no stump, but in Scyphozoa a stump is left 

 which can regenerate a new head. This is what the only differ- 

 ence, often much emphasized, between the formation of a medusa by 

 lateral budding and the formation of a medusa by transverse fission 

 consists in. 



The fact that the sexual cells of Scyphozoa are discharged 

 inwards towards the stomach cavity, and not outwards as in 

 Hydrozoa, is a real difference. But it must be remembered that 

 it is confidently asserted that in many Hydrozoa with rudimentary 

 gonophores the sexual cells originate in the endoderm; and it may be 

 that they always have an ectodermal origin, but that in their first 

 stages they are indistinguishable from ectodermal interstitial cells. 

 Gastral filaments and septal muscles constitute, however, features 

 in which Scyphozoan organization is higher than that of Hydrozoa. 



III. ACTINOZOA 



The great group of the Actinozoa, one of the four primary 

 groups of the Coelenterata, is distinguished from the Scyphozoa by 

 the replacement of the oral cone by an inturned tube of ectoderm, 

 the stomodaeum. 



The eggs of Actinozoa, like those of Scyphozoa, are developed in 

 the endoderm and dehisced into the coelenteron of the parent. In 

 most Zoantharia the embryos pass through the first stages of their 

 development within the body of the mother ; but in a few Zoantharia 

 and apparently in most Alcyonaria, the eggs are discharged through 

 the mouth of the parent into the sea and fertilized there. In this 

 case it is possible to obtain a great many specimens of the same age. 

 But when development takes place within the coelenteron of the 

 mother only a very few specimens of the earliest stages of development 

 will be found in any one individual parent?, since these stages are 

 rapidly passed through. For this reason we select a Zoantharian 

 (Urticina crassicornis}, in which the eggs are discharged before 

 fertilization, as a type for special study in order to illustrate the 

 development of Actinozoa. 



URTICINA CKASSICORNIS 



Urticina crassicornis is a sea-anemone found on the British and 

 Norwegian coasts and its development has been worked out by 

 Appellof (1900). This observer kept the adults living in tanks in 

 the Bergen aquarium until they spawned ; he kept the eggs in 

 dishes of clean sea- water until the larvae hatched out, and these he 

 was able to keep alive until they fixed themselves and metamor- 

 phosed into young sea-anemones. As preservative he used the 

 mixture of corrosive sublimate solution and acetic acid described 

 in Chapter II. 



