Coelenterate Nature of the Sponges. 95 



by modern ideas, finds no sufficient grounds, in what we at 

 present know of these, for ascribing, as I have done, to the 

 common ancestors of the Sponges and Cnidaria radially 

 arranged mesenterial sacs, tentacles with urticating capsules, 

 and indiiFerent aquiferous pores. In opposition to this I 

 might indicate that I have not definitely assumed the exist- 

 ence of the tentacles and urticating capsules, but I say * : — 

 ''''It may he difficult to ascertain lohether the Sponges are or are 

 not retrogressive as regards these organs {i. e. urticating organs) 

 and the tentacles ; but in the attempt to demonstrate the 

 Coelenterate nature of the Sponges this is not of preeminent 

 importance ; " and further, " In case the ancestors of the 

 Sponges ever possessed tentacles and urticating organs it is 

 not difficult to understand how these might have been lost ; " 

 and, finally, " It is indeed not impossible that the Sponges 

 hranched off at a developmental stage of the Coelenterate stem, 

 at which tentacles and urticating organs had not yet been 

 differentiated,^'' 



Hence there remain two points, namely the radiate structure 

 and the canal-system, which chiefly determine me to see 

 Ccelenterates in the Sponges ; and it may perhaps not be 

 superfluous to discuss these characters here in detail. 



In the first place we have to attempt to answer the ques- 

 tion, How did the radiate symmetry of the Coelenterata 

 originate ? The literature that deals with this subject is not 

 considerable. There exists, indeed, a whole series of books 

 and memoirs in which one would expect a priori to find state- 

 ments relating to it ; but they refer to the radiate structure in 

 general, and do not enter upon its origin. Properly speaking 

 Leuckartf alone has spoken in various places in some detail 

 upon this point, and we shall therefore on the whole adopt 

 his guidance in what follows. 



If we imagine a creature of spherical, discoidal, conical, or 

 cylindrical form, swimming (but entirely in the water, not at 

 its surface), this, supposing it to possess a body composed of 

 perfectly homogeneous substance, will always find itself in 

 equilibrium. If we imagine, further, that the substance of 

 the body does not remain homogeneous, that, for example, 

 a heavier part is differentiated, this will place itself either 

 exactly centrally, i. e. axially, or it must form a uniformly 

 developed mantle around the lighter part. Or the substance 



* Zeitschr. fur wiss. Zool. Bd. xxxvii. p. 244, 



t In various places in his ' Jaliresberichte/ and, further, in his tract 

 * Ueber die Morphol. &c. d. wirbell. Thiere,' p. 14, and especially in 

 Bergmann and Leuckart, ' Anat. physiol. Uebers. d. Thierreicns,' pp. 392 

 et seqq. 



8* 



