of the Sjaonges to the Corals. 21 



general. Diversity concerns us more than unity, fact more 

 than theory. It is right to know what the form of a brick is, 

 but it is of more consequence to know what structures a com- 

 bination of them may produce. A mansion and a monument 

 are not necessarily allied because they are both built of brick, 

 nor is the sponge allied to the coral because both may have 

 originated from the same kind of ovum in a similar way. It 

 is the diiferentiation of their respective structures afterwards 

 that is of most importance to the naturalist; and it is precisely 

 on this point that Hackel and myself differ. One would make 

 the sponges go along with the corals, and the other in the 

 direction of the compound tunicated animals. 



But although our premises being different precludes my 

 arguing against Hackel's hypothesis, there are other points 

 in his interesting paper which do appear to me to be directly 

 assailable. 



Thus at p. 8 he states : — " That the essential agreement in 

 the internal organization of sponges and corals, their actual 

 homology, has hitherto been for the most part overlooked is 

 due, among other things, to the fact that the most accurate 

 anatomical investigations of recent times (especially those of 

 Lieberkiihn) took their start from the best-known and most 

 common forms of sponges — viz. the freshwater sponge {Spon- 

 gilla) , which belongs to the group of the true siliceous sponges, 

 and the common sponge {Euspongia)^ belonging to the group 

 of horny sponges. But these very two forms of sponges differ 

 in many respects considerably from the original and typical 

 structure of the entire class, have been in many ways modified 

 and reti'omorphosed by adaptation to special conditions of 

 existence, and therefore easily lead to erroneous conceptions, 

 especially as their investigation is comparatively difficult. 



" On the other hand, among all the sponges, no group ap- 

 pears better fitted to shed full light upon the typical organi- 

 zation and the true relations of affinity of the whole class than 

 the legion of the Calcispongite." 



This recalls to mind the old story in Mavor's ' Spelling- 

 Book ' of the town in danger, when, the different artisans 

 meeting together for a council of defence, the shoemaker stated 

 that " there was nothing like leather." The same, however, 

 may be stated of what I myself am about to assert, which is, 

 that there is nothing like Spongilla for the pm'pose of studying 

 sponge-development. 



As a medallist in the classes of comparative anatomy (under 

 Prof. R.E. Grant) and of human anatomy at University College 

 in 1836-37, as a practical and experimental observer of Spon- 

 gilla in its living state, for many years, when it grew in the 



