122 PORIFERA—SPONCES. 



in a clump, some forming an internal mass, others a complex protective 

 capsule with capstan-like spicules, known as amphidiscs. According to 

 W. Marshall, the life history is as follows : In autumn the sponge suffers 

 from the cold and the scarcity of food, and dies away. But throughout the 

 moribund parent gemmules are formed. These survive the winter, and 

 in April or May they float away from the dead parent, and develop 

 into new sponges. Some become short lived males, others more stable 

 females. The ova produced by the latter and fertilised by spermatozoa 

 from the former, develop into a summer generation of sponges, which in 

 turn die away in autumn and give rise to gemmules. The life history 

 thus illustrates what is called alternation of generations (see p. 55). 

 Interpreted from a utilitarian point of view, the formation of gemmules 

 is a life saving expedient. As Prof. Sollas says, " the gemmules serve 

 primarily a protective purpose, ensuring the persistence of the race, 

 while, as a secondary function, they serve for dispersal." 



All Sponges produce sex cells, which seem to arise from 

 amceboid mesoglcea cells retaining an embryonic character. 

 In the case of the ovum, the amceboid cell increases in size, 

 and passes into a resting stage ; in the case of the male 

 elements, the amceboid cell divides into a spherical cluster 

 of numerous minute spermatozoa. The similar origin of 

 the ova and spermatozoa is of interest. Most sponges are 

 unisexual, but many are hermaphrodite. In the latter case, 

 however, either the production of ova or the production of 

 spermatozoa usually preponderates, probably in dependence 

 on nutritive conditions. 



Development. 



It is not surprising to find that there is great variety of 

 development in the lowest class of Metazoa ; it seems almost 

 as if numerous experiments had been made, none attended 

 with progressive success. 



The minute ovum, without any protective membrane, 

 usually lies near one of the canals, and is fertilised by a 

 spermatozoon borne to it by the water. It exhibits a certain 

 power of migration as in some Hydroids. Previous to 

 fertilisation, the usual extrusion of polar bodies has been 

 observed in a few cases, and is doubtless general. Seg- 

 mentation is total and usually equal, and results in a 

 spherical or oval enrbryo more or less flagellate. This 

 leaves the parent sponge, swiins about for a time, then 

 settles down, and undergoes a larval metamorphosis often 

 difficult to understand. It is peculiarly difficult to bring the 



