of the Sponges to the Corah. 21 



general. Diversity concenis us more than unity, fact more 

 tliiiu tlu-urA'. It is right to kimw what the turni of a hrick is, 

 but it is ot more conseciueiiee to know what strueture.s a ctmi- 

 bination of them may jtnxhiee. 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 tile dirterentiation of their respective structures afterwards 

 that is of must impurtaiice to the naturalist; and it is jnrcisely 

 on this point that iliickel and myself difier. Une 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 difterent precludes my 

 arguing against Hackers 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 

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

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

 (/ ilia), which belongs to the group of the true siliceous sponges, 

 and the common sponge [Eusjjomjia), belonging to the group 

 of horny sponges. But these very two forms of sponges diti'er 

 in many respects considerably from the original and typical 

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

 and retromorphosed by adaptation to special conditions of 

 existence, and therefore easily lead to erroneous conceptions, 

 esj)ecially 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 Calcispongia." 



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 prnpose 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 Span- 

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



