the Sponges to the Corals'. 217 



tlie admission of the water into tlie subjacent intermarginal 

 cavities, e ; on these becoming closed up, on account of irrita- 

 tion or other causes, apertures reappear, not where tlie origi- 

 nal ones obtained, but at a totally different portion of the 

 membrane. This property is essentially Protozoic. Accord- 

 ing to Hackel, the only difference in histological structure 

 existing between the Coelenterata and the Spongiadse is that 

 the representatives of the former possess nematophores or 

 urticating cells, while those of the latter are entirely devoid 

 of them. It must be admitted that this distinction is of 

 itself a very important one, since it demonstrates that the 

 former possess a much more complex degree of organization. 

 But this is surely not all : Hackel seems to have entirely 

 ignored the fact that the tissues of the Coelenterata undergo 

 a still further degree of modification, and assume the form of 

 true unstriated muscular tissue ; and in some of the higher 

 forms (the Ctenophora) even a nervous system has been 

 discovered. 



In the sponges, on the other hand, primitive fibrous or 

 connective tissue is the very higliest degree of differentiation 

 which obtains. 



Lastly, it may be considered an open question whether a 

 sponge-body can lay claim to the rank of distinct and separate 

 individuality, or whether, as in accordance with the views of 

 the majority of modern writers, it must not be regarded as an 

 aggregation of amoebiform animals building up among them- 

 selves a common skeletal support. 



This latter interpretation forces itself strongly upon one's 

 mind when we come to consider the nature of the sarcodic 

 substance lining all the interstitial cavities of the sponge, and 

 spreading itself out upon and investing its horny, siliceous, 

 or calcareous skeleton, which sarcode is capable of resolving 

 itself into masses of unequal size and variable form", of sepa- 

 rating itself from the parent mass and becoming developed 

 into a perfect sponge, or of uniting with it again, or with any 

 other individual of the same species. 



In the same way with the minute sponge-particles lining 

 the passages, each of which is capable of appropriating to it- 

 self the molecules of food brought within its reach ; so that, to 

 borrow a metaphor from Professor Huxley, when treating on 

 Spongilla^ " We must not compare the system of apertures 

 and canals to so many mouths and intestines, but the sponge 

 represents a kind of subaqueous city, where the people are 

 arranged about the streets and roads in such a manner that 

 each can appropriate his food from the water as it passes 

 along." 



Ann. (k Mag. N. Hist. Ser, 4. Vol. v. 15 



