516 B. A. MINOHIN. 



and grows until the gastral cells form a single layer round it. 

 To see the spicules and their cells one must focus the microscope 

 just below the dermal epithelium and external to the gastral 

 layer. In this way I was able to prove to satisfaction that the 

 triradiate spicules, from their first appearance onwards, are 

 formed just as in the adult. This is shown in PI. 39, figs. 

 22 — 26, for falcata; fig. 27, for cerebrum; and PI. 40, 

 figs. 31 and 32, for reticulum. On the other hand, my 

 former statement with regard to cerebrum (1896 [1], p. 51), 

 that the spicule-forming cells commence to secrete the spicule 

 when still on a level with the dermal epithelium, was not 

 confirmed by more extended investigations. This statement 

 was based upon the examination of living specimens, at a time 

 when material was scarce. Working a year later, with abun- 

 dance of material carefully preserved, I was unable to find a 

 single instance in which even the smallest spicules were not 

 covered by the layer of dermal epithelium; and I must therefore 

 retract my former statement, or at least restrict its applica- 

 tion. The origin of the spicule-secreting cells from the dermal 

 epithelium is very easily made out in the larva, both in surface 

 views and sections. I reserve completer proof of this point 

 for a work on the embryology. A point of difference between 

 the young and the adults is found in the relatively large 

 number of irregular spicules in the former. It is, if anything, 

 rather the exception to find the rays meeting at regular angles ; 

 I have even observed instances in which one ray was lacking 

 entirely, the two remaining rays meeting, perhaps, at an acute 

 angle. Later, when the young sponge assumes the adult 

 structure, the spicules formed are quite normal, and the irre- 

 gularity becomes less marked. In spite of their tendency to 

 variation, the first spicules can always be recognised plainly as 

 belonging to the triradiate type, and that only. Monaxons 

 and quadriradiates do not appear, as far as I have observed, 

 till after the osculum is formed and the sponge has begun 

 to grow. 



To summarise the facts here brought forward with regard to 

 the origin of the triradiate systems, we can recognise a scheme 



