Development of the Calcispongiae. 47 



the Sponges to the Coelenterata, and the comparison of the 

 ' water-vascular system' of the former with the l gastro-vascular 

 apparatus ' of the latter, which Leuckart first indicated, has 

 then been demonstrated (?) more circumstantially and firmly 

 established by developmental history in my memoir ' Ueber 

 den Organismus der Schwamme ' &c. I therein proved that 

 a true homology really exists between these two systems of 

 canals, and that the wall of these canals in the Sponges, 

 as well as in the Hydromedusse, Ctenophora, and Corals, is 

 formed from two originally different cell-layers or lamellae — 

 namely from the exoderm, which represents the outer, and 

 the entoderm, whiclr represents the inner germ -lamella of the 

 higher animals. I further demonstrated that these two primi- 

 tive formative membranes show the same characters in the 

 ciliated larva developed from the egg in both the groups of 

 the Coelenterata and Sponges " (p. 214 ; see also p. 33). As 

 we have seen that one of the principal momenta in the deve- 

 lopmental history of the Calcispongiae, the metamorphosis, was 

 not observed but constructed a priori by Hackel, and further 

 that this construction is contradicted by facts, of course the 

 just cited opinion as to the germ-lamellae of the sponges must 

 also be subjected to a thorough revision. 



I will first consider HackePs statements, and then pass to 

 the expression of my own views. Hackel has expounded his 

 theory most completely in the last section of his first volume 

 ("Philosophic der Kalkschwamme "). We find there the 

 following passages : — " If we compare the coarser and finer 

 structural characters of Hydra and Cordylophora .... with 

 the corresponding structural characters of Olynthus* , we can- 

 not but be astonished at the striking agreement which occurs 

 even in minute details " (p. 460). Now in what does this 

 astonishing agreement consist? "1, the simple stomachal 

 cavity Avith buccal orifice; 2, the composition of their stomachal 

 wall of two lamellae, the ciliated entoderm and the unciliated 

 exoderm ; 3, the composition of the entoderm of flagellate 

 cells" (p. 460). The differences cited by Hackel are as 

 follows : — " 1, the constitution of the exoderm, the cells of 

 which in Hydra and Cordylophora develop urticating capsules 

 and neuro- muscular processes, but in Olynthus fuse into the 

 syncytium ; 2, the circlet of tentacles of the former, which is 

 wanting in the latter ; 3, the different origin of the sexual 

 organs."^ It will be seen at once that in the first three points 

 an homology of the entoderm alone can be spoken of, since for 



f~ * By Olynthus Hackel understands a simple, solitary calcareous sponge 

 with double walls (ectoderm and endoderm), and with a spacious sacci- 

 form "stomachal cavity." 



