132 



PHYLUM PORT PER A SPONGES. 



(a) In the small calcareous sponge Sycandra raphanus (Fig. 61), as 

 described by F. E. Schulze, the segmentation results in a hollow ball 

 of cells the blastula. A few cells at the lower pole remain large, 

 and are filled with nutritive granules ; the other cells divide rapidly 

 and become small, clear, columnar, and flagellate. The large granular 

 cells become invaginated, forming what is called a " pseudo-gastrula." 

 This leaves the parent, and the cells forming the lower hemisphere 

 of the embryo become rounded and 

 non- flagellate. The embryo swims for 

 a time actively, but the flagellate cells 

 of the upper hemisphere are invaginated 

 into or overgrown by the large granular 

 cells, and thus what is generally called 

 the gastrula stage results. This soon 

 settles down, on rock or seaweed, with 

 the blastopore or gastrula mouth down- 

 wards, and is moored by amoeboid 

 processes from the granular cells, which 

 likewise obliterate the blastopore. The 

 granular cells lose their granules, for 

 the larva is not yet feeding ; the now 

 internal flagella disappear in the absence 

 of the stimulating water ; a mesoglcea 

 with spicules begins to be formed 

 between the inner and outer layer, 

 probably by migrants from the latter. 

 But this disadvantageous state of affairs 

 cannot last.- Pores open through the 

 walls, the entrance of water enables the 

 inner cells to recover their flagella, and 

 an exhalant aperture is ruptured at the 

 upper pole. The young sponge is now 

 in an Ascon stage, from which, by 

 the outgrowth of the inner layer into 

 radial chambers, it passes into the 

 permanent Sycon form, grows into a 



FIG. 62. Diagrammatic re- 

 presentation of development 

 of Oscarella lobularis. 

 After Heider. 



down. 



Next figure shows folding of inner 

 layer (En.) ; Ec. t outer layer. 



Lowest figure shows radial cham- 

 bers (R.C.); Mesogloea (Mg.); 

 inhalant pore (P.)', exhalant 

 osculum (O.). 



., Free-swimming blastula with cylinder, and becomes differentiated in 

 flagella; G., gastrula settled detail (Fig. 6l). 



(b) In Oscarella (Halisarca) lobularis 

 (Fig. 62), a sponge without any skeleton, 

 the ovum segments equally into a 

 blastula, which is flagellate all over. 

 This free-swimming stage may be in- 

 vaginated from either pole to form a 

 hemispherical gastrula, which settles mouth downwards. Pores, an 

 osculum, and the mesoglcea are formed as before, and the inner layer 

 becomes folded into flagellate chambers. 



The main features of sponge embryology are thus summarised by 

 Minchin : 



"I. The larva is composed of three classes of cell-elements: (i) 

 Columnar flagellated cells, forming the outer covering or localised at 



