PROTOZOA: SPONGIDA. 91 



rior of the capsule thus formed is a mass of protoplasmic cells, which, on 

 the coming of spring, is extruded through the hiliform opening of the 

 capsule into the water, and becomes developed into a young Spongilla. 



In the second or sexual method of reproduction, certain of the sponge- 

 particles or " sarcoids " separate themselves and become nucleolo-nucleated, 

 thus constituting ova. At the same time other sarcoids become motionless, 

 and their contents become molecular, and are finally converted into sper- 

 matozoa. By the rupture of these, and by the consequent contact of the 

 different elements (fig. 30, A), embryos are produced, which are at first 

 ciliated and move about freely, becoming eventually stationary, and de- 

 veloping into new individuals. 



As regards the development of the sponges, the impregnated ovum (fig. 

 30, A) cleaves, by the usual process of "segmentation," into a mass of 

 primitive cells, sometimes containing centrally a primitive and temporary 

 cavity (fig. 30, B). These cells are divisible into two distinct groups, one 



Fig. 30. Development of Calcispongice. A, Ovum in the act of being impregnated by 

 the spermatozoids. B, Free-swimming embryo of Sycon, showing the non-ciliated 

 ectodermal cells, and the ciliated endodermal cells, the latter enclosing a temporary 

 "segmentation-cavity" (a). C, The embryo further advanced, with the ciliated half 

 of the body reduced in size. D, The embryo at a later stage, showing the primitive 

 spicules, and the commencing body-cavity (d). E, Unattached larva, without the 

 skeleton ; the ciliated endoderm has now been withdrawn within the non-ciliated ecto- 

 derm, and the primitive opening into the body-cavity (e) has been formed by invagina- 

 tion. F, Young Sycon, six days old, showing the skeleton, b Non-ciliated ectodermal 

 cells ; c Ciliated endodermal cells. (A is after Haeckel ; B, C, D, E, and F are after 

 Metschnikoff.) 



of which ultimately forms the external layer (ectoderm), whilst the other 

 forms the internal layer (endoderm). As described by Metschnikoff in the 

 embryo of Sycon, these groups of cells at first form the two poles of the 



