26 PROCEEDINGS OP THE ACADEMY OP [1886. 



cells. Thus the most interior regions of the organism are 

 visited by this water current and its contents. 



Of all animal types, therefore, that of the sponge is least pro- 

 tected by defensive appliances. There is nothing in the make-up 

 of the organism to prevent enemies from entering and penetrat- 

 ing to every portion of the body. They cannot be confined to 

 an intestinal tube, and exposed to the action of its digestive 

 juices, as in most organisms, but the whole interior of the body 

 is open to assault. 



Yet the evolution and long-continued existence of the sponge 

 type is evidence that it has possessed some defensive adaptation. 

 This adaptation, in its original phase at least, I take to be the 

 peculiar sj^stem of inhalent and exhalent apertures. It is evident 

 that the entrance of the water currents at the large aperture 

 would greatly facilitate the entrance of the foes of the sponge, 

 since they would be drawn from a distance and carried into the 

 body on these currents. As it is, the currents enter at minute 

 apertures, only adapted to the passage of microscopic food par- 

 ticles, and closing completel}'^ when not in use. The spicules 

 may also act defensively, to prevent any larger creature from 

 forcing an entrance at these oral apertures. 



On the other hand, there is a strong current of outflow from 

 the large aperture, well adapted tO' drive away the foes of the 

 sponge, unless possessed of considerable swimming power. In 

 this fact we seem to have an explanation of the peculiar organi- 

 zation of the sponge. The primeval ancestors of the Coelen- 

 terates probably developed in two directions. In one t}' pe tenta- 

 cles and thread-cells appeared, the oral opening was defended, 

 and the internal cavity took on the function of an incipient 

 stomach. In the other type the simple gastrula form was 

 retained, and water was drawn in by ciliary action through pores 

 in its cellular wall, and discharged at the mouth opening, after 

 yielding its food particles to the individual cells. Thus a multi- 

 tude of minute currents were combined into a single protective 

 outflowing current of considerable strength. 



This formation seems beneficial to the sponge from an aggres- 

 sive as well as from a defensive point of view. The extension of 

 the oral apertures over the whole surface of the body very con- 

 siderably increases the water space from which food particles are 

 drawn in, and thus may add materially to the food supply. As 



