Calcispongiffi in the Animal Kingdom. 251 



of those histological characters which with most certainty 

 separated the ^calephse from the Sponges. Until recently 

 the proposition was current that all Acalephge possess urti- 

 cating organs, and all Sponges are destitute of them. But 

 Eimer* has lately stated that he has found urticating cells 

 also in several species of siliceous sponges (Renierinaj). Con- 

 sequently this differential character also seems to lose its 

 vatue. There would consequently remain as the sole dif- 

 ferential character between Acalephie and Sponges, iho, pore- 

 structure of the latter, on account of which Grant named them 

 Porifera. But, in my previous memoir on the organization 

 of the sponges, I have already pointed out that in many 

 Acalephaj cutaneous pores also occur, which open into the 

 gastro-canal system, and allow water to penetrate into it from 

 without. In the Medusaj such aquiferous apertures have 

 been described by various authors. In the Corals, cutaneous 

 pores, which introduce water from without into the ramifica- 

 tions of the gastro-canal system, appear, from the observations 

 of Milne- Edwards, Kolliker, and others, to be very widely 

 diffused. Still it is very remarkable that these pores appear 

 to be wanting precisely in the lowest Acalephan forms, the 

 Hydroida. Thus, even if we suppose the two lines of the 

 Sponges and Acalephge to separate before the common root, 

 we should have to regard the pore-formation in the two 

 groups as analogous and not as homologous formations, or, 

 more strictly expressed, as liomomorplious but not homophylous 

 structures f. At any rate, however, the boundary between 

 the lower Acalephse (Hydroida) and the lower Sponges appears 

 at present to be so effaced that, at the moment, we cannot 

 establish any single generally applicable differential character 

 between the two groups of the Zoophyta. 



4. The Stem of the Zoophytes {Zoophyta or Coelenterata) . 



In order to facilitate the comprehension of the preceding and 

 following observations, I must here insert a few words as to 

 my conception of the zoophytes in general. In the older zoo- 

 logical systems the animals which are now usually denominated 

 Codenterata are mixed with other lower animals in the section 

 of the Zoophyta, established by Wotton as early as 1552. 

 After Lamarck (1814) and Cuvicr (1S19) it is well known tluvt 

 the Hydroida, Medusas, and Corals were generally placed, 

 together with the Echinodermata &c., in the extremely un- 

 natural division of the radiated animals [Radiata or Radiaria)^ 



* Archiv t'iir niikr. Anat. Bd. viii. 1871, p. 281. 



t I call hoiiiophtjlrj tlie real phylo^eiietically founded homology, in 

 opposition to honioniaqy/ti/, which is destitute of genealogical foundation. 



