MATERIALS FOR A MONOGRAPH OP THE ASCONS. 471 



new facts and observations which willj I trust, bear me out in 

 wliat I have said. 



Studies upon a group of animals naturally fall under four 

 main heads — systematic, structural, developmental, and phy- 

 siological, including under the last-named category rather 

 more than is meant by the term in our universities and medical 

 schools. The first of tliese lines of study is independent of 

 time and place, and can be conducted in any laboratory or 

 museum with material fresh or preserved ; the second and 

 fourth can also be carried on at any time, given a good labora- 

 tory and aquarium and abundance of healthily living speci- 

 mens; the third alone, that is the embryology, must be studied 

 at the right season as well as in the right place, and the 

 observer must be prepared when the season comes to lay aside 

 other work for the time and devote himself to it. Thus, 

 although I liave been occupied for some time past with the 

 histology of Ascons, the embryology has always been regarded 

 by me as having a first claim on my time, and in consequence 

 my histological studies have been disjointed, and are still very 

 far from completion. At present my embryological material 

 is nearly complete, and I hope before long to commence the 

 publication of studies upon it ; but as some time must elapse 

 before a large quantity of material, comprising the whole 

 developmental cycle of a number of species, can be worked 

 through, it seemed best to bring out at once an account of 

 some histological points in which ray results approach to 

 finality. The formation of the spicules so characteristic of 

 these sponges has long engrossed my attention, and I trust 

 that the descriptions and figures to follow will be held to sub- 

 stantiate my claim to have made out the main points in the 

 morphology of the triradiate and quadriradiate spicules in 

 the genera characterised by having the rays of the triradiate 

 systems meeting at equal angles, — that is to say, the genera 

 for which in my recent revision of the system I have used the 

 names Clathrina, Gray, and Ascandra, Haeckel. 



Even with regard to the development of the spicules I am 

 well aware that I still have much to do. On the one hand^ I 



