120 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



when the sponge shrinks up under unfavourable conditions the 

 shrinkage is brought about by the contraction of the dermal epithelial 

 cells. Dotted about at intervals in the dermal epithelium are modified 

 cells known as porocytes (Fig. 55, pc). Each of these^ instead of being 

 thin and plate-like, extends inwards throughout the thickness of the 

 dermal layer and, emerging between the choanocytes, comes into direct 

 relation with the water in the cavity. When the sponge is fully expanded 



the porocyte concentrates its proto- 

 plasm peripherally so that it forms 

 a tubular sheath round an axial 

 cavity which opens on the one hand 

 on the external surface of the sponge 

 and on the other into the gastral 

 cavity. These temporary intracellular 

 openings through the body-wall of 

 the sponge are the pores (Fig. 55, p) 

 and it is through them that there 

 takes place the indraught of water 

 necessary to replace that which 

 passes out through the osculum. 



Other cells of the dermal epi- 

 thelium become modified as sclero- 

 blasts. These take on an amoeboid 

 character, wander into the jelly, and 

 there settle do\yn to form the spicules 

 which compose the skeleton of the 

 sponge. These spicules are needles of 

 calcium carbonate with an axial core 

 and thin external sheath of organic 

 material. The first formed spicules 

 are simple needles (monaxon spicules — 

 Fig. 58, A, two lower figs.) but later 

 on there are formed numerous three-rayed (triradiate) spicules each of 

 which represents a group of monaxon spicules radiating from a point 

 (Fig. 58, A, upper fig.). 



The mode of formation of these compound triradiate spicules is 

 characteristic. Three scleroblasts approach one another and arrange 

 themselves in trefoil fashion. The nucleus of each cell divides, and 

 between the two nuclei a fine needle-like spicule makes its appearance. 

 The three spicules become continuous centrally so as to form the three 

 rays of the compound triradiate spicule. Each ray lies between two 



Fig. 55- 



Diagram illustrating the structure of an 

 Ascon as seen in a transverse section, a, 

 Amoebocyte ; D, dermal layer ; d.e, dermal 

 epithelium ; j, jelly ; p, pore ; pc, porocyte ; 

 sc, scleroblasts ; sp, spicules. 



