120 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



a. 



d.e. 



the sponge shrinks up under unfavourable conditions the 



, about by the contraction of the dermal epithelial 



about at intervals in the dermal epithelium are modified 



porocytes (Fig. 55, PC). Each of these, instead of being 



, m ,l plate like, extends inwards throughout the thickness of the 



(il . rma l i a merging between the choanocytes, comes into direct 



on \s ith 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 down 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 lormed numerous three-rayed (triradiate) spicules each of 

 \\hieh represents a group of monaxon spicules radiating from a point 



\. upper ti 



The mode of formation of these compound triradiate spicules is 



tic, Three scleroHasts approach one another and arrange 



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



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



Tin- three spiculex heroine continuous centrally so as to form the three 



: the impound triradiate spicule. Each ray lies between two 



i ro. 55. 



:u ilhHtr.itiiiL; tin- structure of an 

 Ascon as MM in .1 transverse section, a, 



! '. >lri iii.il l.ivrr ; </.(', <lrnn.il 



fpitli.-liniii ; j, ji-lly : />, pore; pc, porocyte; 



