CHAPTER V 



PORIFERA. 



ALTHOUGH within the last few years greater advances have pro- 

 bably been made in our knowledge of the development of the 

 Poritera 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 completely 

 worked out is Sycandra raphanus, one of the Calcispongiae (Metschni- 

 koff, 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 somewhat 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 intermediate in position between the two 

 first planes of segmentation. Eight equal segments are thus formed, 

 each of which has the form of a pyramid. All the segments are 

 situated in a single tier, and are so arranged as to give to the whole 

 ovum the form of a flat cone, the apex of which is formed by the 

 pointed extremities of the constituent segments (fig. 63 B). The 

 apices of the segments do not however quite meet, but they leave a 



B. E. 8 



