432 EICHARD EVANS. 



spicule, it is destined to change in character, and to become a 

 cell with a granular nucleus. 



The spicules when they have grown somewhat in size show 

 very clearly an axial thread, which traverses their longitudinal 

 axis from end to end, but which cannot be seen in the early 

 stages of their development. The thread in question appears 

 to consist of some form of organic material which lies in the 

 canal of the spicule, and which stains red with fuchsin S near 

 the broken ends of pieces of spicules. The stain seems to 

 penetrate into the canal from the broken end of the spicule 

 and fades away gradually, showing clearly the difference 

 between the stained and the unstained portion of the thread. 



The Microscleres. — These spicules are not formed so 

 early in the development as the megascleres. However, they 

 are found soon after fixation, — for example, the spicules re- 

 presented in fig. 37 a have been drawn from a specimen which 

 still retained the flagellated layer almost complete on the 

 upper surface. They are not developed in cells with vesicular 

 nuclei, but in cells with granular nuclei, that is in those cells 

 which give rise to the flat epithelium of the sponge. And, 

 moreover, they are often seen inside the cells of the flat epi- 

 thelium, two cells of which are represented in fig. 37 h, one 

 containing a flesh spicule. 



It is seen from the above description of the development of 

 the spicules that cells belonging to two out of the three 

 classes found in the youngest larva, i.e. type A, are capable 

 of secreting spicules. 



Appendix D. 



On the Division of Collar-cells and the Multiplica- 

 tion of Chambers. 



There occurs in Mr. Minchin's able paper on the position of 

 sponges in the animal kingdom the following statement : — 

 "The fact remains, that both the multiplication of collar-cells 



