230 THE ZOOLOGIST. 



are formed from the former ; the latter does not change, and sur- 

 rounds the ripening sperm-ball. The mature spermatophores are 

 often found in the walls of the ciliated chambers. In the Silicea 

 no such structures are observed. The sperm mother-cells form by 

 continued division sperm -balls without covering cell. The sper- 

 matozoa lie radiallj' in the extended wall of the sperm mother- 

 cell. In Aplysilla and many other sponges the sperm-balls 

 accumulate in certain parts of the body, particularly in the 

 trabeculse and membranes, which are spread out in the basal 

 or central lacunae. They are often surrounded by a peculiar 

 endothelium, clothing the cavities in which they are contained. 

 This endothelium consists of irregular, flat cells, which lie in 

 several layers one over the other. Farther outward these cells 

 become more irregular, and appear to be separated by layers of 

 intercellular substance, becoming very similar to the common 

 connective-tissue cells. At the time of ripening, the spermatozoa 

 pass into the canals and swarm out through them. 



The ova are also derived from wandering cells. As they 

 mature, they lose their mobility, increase in size, and become 

 spherical, while the nucleus increases in size. The ripe ova are 

 destitute of a thick cell-wall, and lie scattered or in groups in 

 the mesodermal ground substance. They are often surrounded 

 by endothelial capsules similar to those of the sperm-balls. In 

 some cases the ovum appears to be attached by a special 

 peduncle — a differentiated endothelial cell — to the capsule. 



In the hermaphroditic sponges the ova and spermatozoa, to 

 prevent self-fertilisation, do not mature at the same time. They 

 are fructified within the body of the mother, where the first stages 

 of development take place. 



The most interesting and the least known organ system in 

 sponges is their nervous system. The first person who pointed 

 out that the sponges were sensitive was no other than Aristotle. 

 A few years ago Prof. Stewart demonstrated Palpocils of Grantia 

 at a meeting of the Royal Microscopical Societj', but did not pub- 

 lish a description of them at the time. Subsequently I described 

 certain elements in Calcareous Sponges, which I considered as 

 nervous elements, and since then I have found similar sensitive 

 cells in a number of species belonging to various groups. 



Recently also Prof. Stewart has published a figure of his 

 Palpocils, and as he has courteously allowed me to examine his 



