106 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 Afetazoa. The cells of one hemisphere then become more 
numerous and acquire flagel/a, 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 amphiblastula. 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 ScHuULzE.) 
Flagellate Cells. 
ES lls. 
Central Cavity. Granular Cells 
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 amceboid condition and 
withdrawing their flagella. Ina manner little understood the amceboid 
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 :— 
. Total equal segmentation to blastula larva. 
. Differentiation into amphiblastula. 
. Invagination of flagellate half into granular half. 
. Fixation and quiescent amceboid stage. 
. Differentiation of ascon stage. 
. 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. 
aunip WN 
