122 



ZOOLOGY 



year; they are set free by the decay of the part of the parent 

 sponge in which they are developed, and fall to the bottom. In 

 spring the contained mass of protoplasmic matter reaches the 

 exterior through an aperture in the wall of the gemmule, and 

 develops into the adult form. 



All Sponges multiply by a sexual process — by means of male 

 cells, or sperms, and female cells, or ova. These are developed 

 from certain of the amosboid wandering cells of the mesogloea, 

 which take up a special position, usually immediately below the 

 collared cells of the endoderm. Ova and sperms are developed in 

 the same Sponge, but rarely at the same time. The amoeboid cell 

 destined to form sperms divides into a number of small cells, giving 

 rise to a rounded mass of sperms. The latter, when mature, have 

 oval or pear-shaped heads and a long tapering appendage or tail. 

 Each amoeboid cell destined to form an ovum enlarges, and 



Fig. 91.— Various forms of sponge spicules. (Prom Lang's Text-Booh.') 



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 Olyntlius, the development 

 has been followed out in the case of Clathrina hlanca. 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 character — 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 archococytcs, some of which form the repro- 



