CHAPTER V. 



PORIFERA. 



ALTHOUGH within the last few years greater advances have 

 probably been made in our knowledge of the development of the 

 Porifera than of any other group, yet there is much that is still 

 very obscure, and it is not possible to make general statements 

 applying to the whole group. 



Calcispongiae. The form which has so far been most com- 

 pletely worked out is Sycandra raphanus, one of the Calcispon- 

 gise (Metschnikoff, Nos. 132 and 134, F. E. Schulze, Nos. 139 

 and 142), and I shall commence my account with the life-history 

 of this species. 



The ovum in Sycandra as in other Spongida has the form of 

 a naked amoeboid nucleated mass of protoplasm. From the 

 analogy of the other members of the group, there is no doubt 

 that it is fertilized by a male spermatic element, though this has 

 not as yet been shewn to be the case and the changes which 

 accompany fertilization are quite unknown. 



The segmentation and early stages of development take 

 place in the tissues of the parent. The segmentation is some- 

 what peculiar, though a modification of a regular segmentation. 

 The ovum divides along a vertical plane, first into two, and then 

 into four equal segments. But even when two segments are 

 formed, each of them has one end pointed and the other broader. 

 The pointed ends give rise to the ciliated cells of the future 

 larva, and the broad ends to the granular cells. Instead of the 

 next division taking place, as is usually the case, in a horizontal 

 (equatorial) plane, it is actually effected along two vertical planes 



