124 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



Medusa?). Accordingly we have to distinguish different forms of 

 alternations of generations, which have genetically a different origin 

 and explanation. 



The latter form of alternations of generations resembles metamor- 

 phosis ; and we have in most cases to explain it as having arisen 

 in the following way : The asexual form corresponds to a lower 

 stage in the phylogenetic history, from which it has inherited the 

 capacity of asexual reproduction, while the sexual reproduction belongs 

 entirely to the higher form. To take as an example the alternation 

 of generations of the Scyphomedusre. The animal is hatched as a 

 free-swimming ciliated planula (gastrula with closed blastopore) (fig. 

 113 a). After a certain time it fixes itself by the pole of its body, 



d 



FIG. 113. Development of the planula of Chrysaora to the Scyphistoma stage, with eight 

 arms, a, Two layered planula with a narrow gastric cavity ; b, the same after its 

 attachment with just-formed month (O), and commencing tentacles ; c, four-armed Scy- 

 phistoma polyp ; Cslc, excreted cuticular skeleton ; d, eight-armed Scyphistoma polyp 

 with wide mouth ; M, longitudinal muscles of the gastric ridges ; Cslc, excreted cuticular 

 skeleton. 



which is directed forward in swimming, and acquires at its free ex- 

 tremity a new mouth, round which 1, 2, 4, 8, and finally 16 long 

 tentacles soon make their appearance ; while the broad oral region 

 projects as a contractile cone (fig. 113 b, c, d). Inside the gastric 

 cavity there project four gastric ridges with longitudinal muscular 

 bands extending from the foot or point of attachment to the base of 

 the oral cone. When the polyp, which has now become a Scyphis- 

 toma, has under favourable conditions of nutrition reached a certain 

 size (about 2 to 4 mm.), ring-like constrictions are formed at the 



