1886.] NATURAL SCIENCES OF PHILADELPHIA. 27 



the microscopic creatures which serve the sponge for food are 

 somewhat thinly disseminated, this extension of the space swept 

 hy the inflowing currents is certainly an advantage. 



These remarks, however, are intended to apply principally to 

 the primitive sponge. In the earliest stage of this type of 

 organism it probably lacked the spicular and fibrous defenses 

 now possessed, and was but a step above the simple gastrula. At 

 this remote era, however, it is probable that muscular action had 

 not yet been evolved, and that cilia formed the only agents of 

 animal motion. Against creatures swimming by aid of cilia the 

 water current of the sponge must have been fully efficacious as a 

 defense. The development of the fibrous framework, and of 

 the pointed spicules, was probably a subsequent adaptation to 

 meet new dangers, after muscular organs of motion had been 

 evolved. At present prowling animals have no difficulty in 

 making their way to the interior of the sponge. The spicule is 

 now probably its most efficient defense, and the continued exist- 

 ence of the sponge tj^pe shows that it is sufficiently defended 

 against such animals as might seek to prey upon its protoplasmic 

 substance. 



I may oflTer, in conclusion, a further application of the hypoth- 

 esis presented in my former paper. The interaction of attack 

 and defense presents a feature of distinction between animals and 

 plants, in addition to those usually recognized. This distinction 

 I ma}^ briefly indicate. 



If we consider the defensive appliances of animals from a 

 general point of view, they may be reduced to two categories. 

 They are either mechanical or motor. These exist together, but 

 to the extent that the one method is developed, the other remains 

 undeveloped. If we take such an animal as the oyster, for 

 instance, we find that its defense is almost wholly mechanical. 

 Its only defensive motion is one of withdrawal within the shell. 

 If we take man, the defence is almost wholly motor. Scarcely a 

 trace of mechanical defense exists. In my former paper I spoke 

 also of mentality as a defensive attribute. But the defensive 

 action of the mind yields really a motor effect. It simply pro- 

 duces more intricate and diversified motions, both in attack and 

 defense, than those displayed by unintelligent animals. 



If we examine the whole range of the animal kingdom, we find 

 every phase of combination of mechanical and motor defense, the 



