Calcispongife in the Animal Kingdom. 249 



muscular processes, whilst in Olynthus (and Archispongia ?) 

 they coalesce to, form the syncytium; 2, the circlet of ten- 

 tacles of the former, which is wanting in the latter ; 3, the 

 different origin of the sexual organs, in the former in the 

 exoderm, in the latter in the entoderm. This last difference 

 appears to be of great importance. But even within the 

 group of the Acalephaj, according to the statements of many 

 observers, the sexual cells originate in some in the exoderm, 

 in others in the entoderm. I shall revert to this, particularly, 

 hereafter. On the other hand, the want of the circlet of 

 tentacles in the Sponges is of no significance, as even in the 

 Hydroida this does not appear at first, and is wanting in 

 many Hydroid forms (Siphonophora). The difference in the 

 formation of the exoderm appears to be of more importance ; 

 but even this is to be regarded as a secondary histological 

 differentiation of the two divergent groups. 



At any rate, these .differences in anatomical structure be- 

 tween the simplest Hydroida and the simplest Sponges appear 

 of quite subordinate significance, when we place in the oppo- 

 site scale the weight of the above extremely important and 

 essential agreements. This weight, moreover, is considerably 

 augmented if we compare the ontogeny of the two groups. 

 Hydra itself does not come first into consideration in this case, 

 because its primitive ontogeny appears evidently to be strongly 

 modified, and effaced and falsified by secondary adaptations. 

 On the contrary, the ontogeny of Cordylopliora^ which per- 

 fectly agrees with that of Olynthus (see Schulze, I. c. pp. 38— 

 41, pi. V. figs. 1-8), is of the greatest importance. The 

 planula, which originates from the morula, and the plano- 

 gastrula, which originates from the planula, are perfectly 

 similar in the two animals ; even the minute structure of the 

 two layers of cells, or germ-lamellaj, which bound the sto- 

 machal cavity of the ovate ciliated larva is in striking agree- 

 ment — tlie small, slender, cylindrical flagellate cells of the 

 exoderm, and the large, non-ciliate, rounded-polyhedral cells 

 of the entoderm*. 



From this perfectly accordant ontogeny and anatomy of 

 Olynthus and Cordylophora follows with perfect certainty 

 that conception of the position of the sponges in the animal 

 kingdom which I put forward in 18G9 in my memoir " On the 



* It is true that in Corch/lophnra, the breaking out of the stomachal 

 cavity and the formation of the mouth-aperture takes place only after the 

 planogastrula has attached itself, and passed into the Asciila-form ; but 

 even m many constantly astomatous sponges the gastrula appears not to 

 be developed, and the planogastrula becomes directly converted into the 

 Clistolynthus, whilst in Olynthus it passes previously into the gastrula. 



