302 DR. G. P. BIDDER ON THE 



external epithelium within the zone of the currents entering the dermal pores, 

 and once they are in this zone, it is desirable by fur of spicules and by 

 inpouching of afferent canals to shelter the still swimming visitor from the 

 chance surge of a current over the surface. On the other hand, it is possible 

 that the amcebuliform characters of Calcinean spermatozoa, in so far as it is 

 general, has limited the modifications of Canal-System in this group to those 

 which facilitate entrance to a " minute wandering cell " fallen on the outer 

 surface and creeping towards an odoriferous pore*. 



Whether or not there be this difference between the male elements of 

 the two groups, I venture to propose for serious consideration that in sexual 

 sponges the excreted granules of the external surface and pore-cells have an 

 important function in inducing chemiotaxis in the spermatozoa from other 

 individuals of the species. This would be useless to sponges in a tideway, 

 which must take their chance of extraneous fertilisation, but live in the best 

 circumstances for nutrition and asexual growth. It would be of the utmost 

 advantage to a solitary sponge in a deep cave of still water, where unfavour- 

 able conditions make the stimulation of a foreign gamete most desirable. 



It would be strange if, in all the hundreds and thousands of species 

 recognised by some modern authors, the ova of ever}' sponge should refuse 

 absolutely to unite with the spermatozoa from a sponge, the size of whose 

 microscleres proved that his place is in a different paragraph of the page. 

 I am not aware of any evidence whatsoever as to the limits or extent of 

 crossing in sponges. A priori, hybridisation must play a great part in the 

 characters we laboriously tabulate. 



It is strange iu any case to consider that the selective power of a sperma- 

 tozoon, adrift in the North Sea, must often be as highly developed as that of 

 a Red Admiral butterfly. Somewhere in that infinitesimal body we must 

 picture the mechanism for this reflex, as well as the heritages to be trans- 

 mitted to every part of its giant offspring, the zygote. 



Postscript. (September 1920.) 

 Spermatozoa of Sycon raphanus and fertilisation of the ovum. 



Professor Dendy and Mr. Gatenby each suggested to me that, if the dolly- 

 cells are spermatozoa, my drawings may represent their entrance into the 

 gonocytes instead of their exit from them. 



This suggestion I now accept, as also that of Mr. Gratenby, that the stained 



* Since this paper was read, Professor Dendy has kindly shown me the collar-cells of 

 Leucosolenia stolonifer, Dendy (1891), and I have come to the conclusion that this most 

 interesting species is an aberrant Calcinean (see 1913 b, p. 718). It has assumed, however, 

 not only the outward form of Leucosolenia, but also the afferent funnels ; therefore, if my 

 view of its position be correct, it bears evidence against the above suggestion, and in favour 

 of the advantage of the funnels being a hydro-mechanic advantage. 



