io6 PORIFERA. 



by an incoming spermatozoon, after which it withdraws into the body 

 of the sponge and undergoes segmentation. 



The spermatozoa are produced from gonocytes apparently similar to 

 the female cells. A male gonocyte divides up into a great number of 

 spermatozoa which are discharged into the water. 



The ovum segments totally and equally (Chapter V. ) to produce a 

 hollow sphere of cells, each of which in some other sponges bears a 

 flagellum. This stage has been compared with the blastula larva of 

 other Metazoa. The cells of one hemisphere then become more 

 numerous and acquire flagella^ whilst those at the other hemisphere 

 remain few, large and granular. The larva escapes from the parent 

 and swims freely. This larva is only found in sponges and only in 

 certain of them ; it is called an amphiblasttda. The granular cells then 

 grow round the flagellate cells, forming a sort of invagination of the 



Fig. 40. — Amphiblastula Larva of a Calcareous Sponge. 



(After ScHULZE.) 



Flagellate Cells. 



r. 1^ • / ».• s- .'!;•> .«>.^.-.^ ~ Granular Cells. 



Central Cavity. • >. . . . 



latter, and the larva settles down by the free edge of the granular cells 

 upon a foreign body. A sort of metamorphosis then appears to take 

 place, the cells being largely reduced to an amoeboid condition and 

 withdrawing their flagella. In a manner little understood the amoeboid 

 cells of the body of the sponge are produced between the two layers. 

 The nutritive granules in the outer layer are slowly consumed during 

 this process. 



The osculum then opens at the apex, and pores are formed through 

 the sides. The inner layer then becomes flagellate. At this stage the 

 whole internal paragastric cavity is lined by flagellate cells. As soon as 

 the radial canals are produced the collared cells lining the paragastric 

 cavity become pinnacocytic, and the young sponge comes to resemble 

 its parent. The development is thus : — 



1. Total equal segmentation to blastula larva. 



2. Differentiation into amphiblastula. 



3. Invagination of flagellate half into granular half. 



4. Fixation and quiescent amceboid stage. 



5. Differentiation of ascon stage. 



6. Modification into sycon (sycandra). 



Sycandra may also reproduce asexually by budding. A part of the 

 body-wall protrudes and acquires an osculum. It then separates from 

 its parent, or the bud may remain in connection with it and form a 

 colony. 



