34 THE SPONGES 



amoeba-like movements, ''wandering cells" (Archcvocytes), and 

 " capable of first performing elementary functions of digestion, dis- 

 tribution, and probably excretion, and later of becoming germ cells." * 



The reproductive cells, the ova and spermatozoa, are also 

 found in the mesoderm, the ova commencing their existence as cells 

 closely resembling the amoeboid ones, and chiefly distinguished 

 by their large bladder-like nucleus and its large round nucleolus ; 

 while the oval-headed slender-tailed spermatozoa occur in globu- 

 lar clusters or sperm-balls, each the product of a single cell. Both 

 ova and spermatozoa may be developed in the same Sponge, 

 though rarely at the same time, so that probably in most cases 

 the ova are fertilised by spermatozoa from another Sponge. 



Reproduction in the Sponges is effected by sexual or asexual 

 methods. In the latter, multiplication takes place by the pro- 

 duction of internal buds in the shape of groups of cells called 

 gemmules, which eventually become detached from the parent 

 Sponge and develop into new individuals. In the sexual process 

 the ovum, after impregnation, usually becomes enclosed by certain 

 neighbouring cells, which form a brood-capsule around it, and in 

 this enclosure, and still within the parent Sponge, passes through 

 the earlier stages of its development. Finally it escapes as a 

 ciliated, somewhat oval-shaped larva, and by means of its cilia 

 swims about and for a short time leads an active existence, but 

 eventually becomes fixed to a base and develops into the sedentary 

 adult stage. 



The majority of the Sponges possess a skeleton or supporting 

 framework, composed of material secreted by the Sponge itself, 

 and in some instances supplemented by an admixture of sand, 

 the tests or shells of foraminifera and radiolaria, or the spicules 

 of other Sponges. The skeleton may be composed of calcareous 2 

 or siliceous 8 spicules, or of an organic substance called spongin* 

 which occurs either as a cementing mass which glues the spicules 

 together into a more or less definite system of skeletal fibres, or 

 as an elastic felt-like tissue destitute of spicules, as, for example, 

 in the common Bath Sponge, the skeleton of which is composed 

 entirely of a soft, perforated mass of spongin. 



1 Professor Minchin in Lankester's "Treatise on Zoology." 



2 Calcium carbonate, lime. 



Silica, silex, flint. 



* Spongin, a substance allied to silk in its chemical composition. 



