FORM OF BODY OF CCELENTERATA. 99 



mouth-bearing portion of Scyphostoma is gradually nipped off from 

 the rest of the body (Fig. 34, 4). As the body grows the new por- 

 tions, which are formed towards the aboral pole, become separated 

 metamerically (Strobila, Fig-. 34, 5), and all are developed on a 

 Medusa type. The polyp-body is thus divided into a number, often 

 a large number, of Medusa?, which gradually break off (Ephyra 

 form), and when they are free become more developed. 



This process, which has been observed in Cephasa, Aurelia, and 

 Cassiopeia, does not obtain in Pelagia, the ova of which are con- 

 verted into swimming larva?, which become young Medusa), without 

 passing through the polyp stage. The development of Pelagia is 

 therefore compressed into a few stages, while in the others it is 

 extended over a large series of forms, and is a more complete 

 repetition of the palasontological development. The polypoid must 

 be regarded as the initial stage, and was followed by the 

 gradual metamorphosis of the polyp into a free Medusa. On 

 this hypothesis, the strobilation of Scyphostoma and the consequent 

 development of a immber of Medusas, appears to be a secondary 

 process, which could only come about gradually, and after the whole 

 polyp-body had ceased to be converted into a Medusa. It is clear, 

 from the growth of the polyp, while it is passing into the Strobila, 

 that an important part must be played by the nutritive relations of 

 the Scyphostoma stage, in giving rise to the Strobila-form, or, in 

 other words, in producing the Medusas by gemmation ; the whole 

 phasnomenon, therefore, appears to be causally related to the nutri- 

 tion o'f the Scyphostoma. By the gemmation of Ephyras, i.e. of 

 young Discophora, from the body of the Strobila, an asexual mode 

 of multiplication is interposed in the developmental process of the 

 Medusas ; and thus we have brought about one form of the so-called 

 alternation of generation. 



By the Scyphostoma form the Medusas are closely related to 

 the Calycozoa, which appear to be derived from them. The body, 

 which is attached by a short stalk, is widened out like an umbrella, 

 and agrees, as to its axes, with the Scyphostomas, and their 

 descendants. In many points they also present relations with the 

 Anthozoa. The Calycozoa, therefore, present us with a very 

 important intermediate form, which has been continued on, with 

 relatively few modifications, from the ancestral form common to 

 several large divisions of the Acalephas. 



§ 77. 



In the Anthozoa the primitive form of the body is exactly the 

 same as that of the other Coelenterata ; and even the earliest stages 

 of the fixed planula present no essential differences. The appear- 

 ance of tentacles and the subsequent internal differentiation give 

 rise to various differences, the first of which affects the fundamental 

 number of the secondary axes of the body. In some only four 

 tentacles appear (Tetractinia), in others six (Hexactinia), and finally, 



