GEMMULES OF EPHYDATIA 



177 



Ephydatia fluviatilis normally produces not only free-swim- 

 ming larvae of sexual origin, but also internal gemmules arising 

 asexually. These bodies appear in autumn, distributed through- 

 out the sponge, often more densely in the deeper layers, and they 

 come into activity only after the death of the parent, an event 

 which happens in this climate at the approach of winter. 



Weltner^ has shown that on the death and disintegration of 

 the mother sponge some of the gemmules remain attached to the 

 old skeleton, some sink and some float. Those which remain 



Fig. 71. — Portion of the skeletal frame- 

 work of JS. flimiatilis. a, Main 

 fibres ; 6, connecting fibres. (After 

 Weltner.) 



Fig. 72. — Spicules of -E.^M^'^a^i7^s. a.h.c, 

 Oxeas, spined and smooth ; d.e, amplii- 

 discs, side and end views. (After 

 Potts.) 



attached are well known to reclothe the dead fibres with living 

 tissue. They inherit, as it were, the advantages of position 

 which contributed to the survival of the parent, as one of the 

 selected fittest. The gemmules which sink are doubtless rolled 

 short distances along the bottom, while those which float 

 have the opportunity of widely distributing the species with 

 the risk of being washed out to sea. But even these floating 

 gemmules are exposed to far less dangers than the delicate free- 

 swimming larvae, for their soft parts are protected from shocks 

 by a thick coat armed with amphidiscs. 



The gemmules are likewise remarkable for their powers of 



1 Arch. Naturg. lix. 1893, p. 246. 

 VOL, I N 



