in 



PHYLUM AND CLASS PORIFERA 



123 



eventually assumes a spherical form. After a sperm has penetrated 

 into its interior and effected impregnation, the ovum usually 

 becomes enclosed in a brood-capsule formed for it by certain 

 neighbouring cells, and in this situation, still enclosed in the parent 

 Sponge, it undergoes the earlier stages of its development. The 

 boring Sponge, Cliona, is the only one, so far as known, in which 

 the early stages of development are passed through externally. 



In all known cases there is a free-swimming ciliated larval 

 stage ; but the form assumed by the larva differs profoundly in 

 different Sponges. Of the simpler types of calcareous sponges 

 with a structure resembling that of the Olynthus, the development 

 has been followed out in the case of Clathrina blanca. In this 

 sponge segmentation is followed by the formation of an oval 

 blastula, the wall of which consists of a single layer of cells all 

 alike in kind elongated, columnar, and flagellate. At one 

 pole of the blastula is seen a pair of cells which are of a different 

 character, being large, 

 rounded, and granular. 

 These are destined to 

 give rise to the archceo- 

 cytes, some of which form 

 the reproductive cells. 

 Certain of the flagellate 

 cells then withdraw their 

 flagella and pass into the 

 internal cavity, becoming 

 amoeboid. Soon a large 

 number of these amoe- 

 boid cells come to fill 

 up a great part of the 

 cavity of the larva, which now passes into a stage corresponding 

 to the planula larva of the Coelenterates (Section IV). This is the 

 larval form known as the parenchymula. The parenchymula 

 (Fig. 94) consists of three kinds of cells : (1) an external layer 

 of flagellate cells ; (2) an inner mass of amoeboid cells ; (3) the 

 two posterior granular cells. In this condition it becomes fixed, 

 and develops into the form of a flat plate with an irregular outline. 

 Most of the amoeboid cells now migrate to the outer surface, passing 

 between the flagellate cells and then becoming arranged outside 

 them to form the ectoderm. The flagellate cells now form an irregular 

 mass together with a number of non-flagellate cells derived from the 

 ectoderm, which are destined to give rise to the porocytes. A 

 cavity appears in the mass, and becomes surrounded by a layer of 

 porocytes. The cavity increases in size, and is soon seen to be 

 bounded not by the porocytes alone, but in part also by flagellate 

 cells. Subsequently the flagellate cells come to form the entire 

 boundary of the cavity, the porocytes passing outwards to become 



FIG. 92. Various forms of sponge spicules. 

 (From Lang's Text-Book.) 



