and their Relationshij) to the Corals. 11 



(flue or stomach) which opens outwards by a single large ori- 

 fice (osculum or mouth). From this cavity canals issue in all 

 directions, which traverse the body-wall, and finally open on 

 their surface by the cutaneous pores. If, on the other hand, 

 we compare a sponge-stock (e. g. S>/codendrum, Spon(/illa) and 

 a coral-stock (e. g. DendrophyUia^ Gorgonia)^ we find in like 

 manner, in both, a nutrient canal-system of the coenenchjTna 

 or ca?nosoma, wliich })laces the cavities of the individual per- 

 sons in communication with each other. 



The diflx'i'cnce in the direction of the current of water which 

 is usually admitted in the two classes is a matter of perfect 

 indifference in this close morphological comparison. Even if 

 this difference was really constant, general, and thoroughgoing, 

 it would not be capable of invalidating our notion of the ho- 

 mology of the canal-system in the body of the sponge and 

 coral. The difference in the circulation of the nutrient stream 

 of water in the two classes of animals would merely prove that 

 no physiological comparison, no analogy^ exists between the 

 individual parts of the vascular system, but that this has 

 rather been lost by adaptation to different conditions of nutri- 

 tion. But by this our morphological comparison of the corre- 

 sponding parts, their homology^ which we must ascribe to 

 inheritance from common ancestors, is in no way affected. 

 But when we have to giasp the true relation of affinity of two 

 groups of animals, we must consider only their actual homo- 

 logies, i. e. those similarities arising from common inlieritance, 

 which alone constitute the true guiding-star in every compara- 

 tive exposition. On the other hand, we must leave entirely 

 out of consideration the analogies which depend upon mere 

 adaptation^ because these are much better fitted to obscure 

 and conceal than to illuminate and clear up this relation of 

 affinity. 



But it must be pointed out tbat this contrast in the direction 

 of the cun-ent of water, which is almost universally assumed 

 to occur in the vascular system of the sponges and corals, and 

 regarded as without exception, is by no means an absolute 

 and unfailing one. Miklucho has already shown that in a 

 gi-eat many sponges the mouth or osculum by no means per- 

 mits only the outflow, but also the inflow of water. I have 

 repeatedly convinced myself, by my own observations, of the 

 correctness of this assertion. Consequently the mouth in 

 many sponges, just as in the corals, serves for both the recep- 

 tion and expulsion of the water and the nutritive constituents 

 contained in it. 



For the right understanding of these relations, those sponges 

 which have no cutaneous pores at all, and in which the sole 



