52 M. E. Metschnikoff on the 



segment of a sphere, which is afterwards invaginated to con- 

 stitute the entoderm. Of the four larvse described and figured 

 by Hackel, that of Sycyssa Huxleyi is most nearly allied to 

 the Sycon-lsa-vee, although the former is strikingly distinguished 

 by the presence of a layer of spherical cells lining the internal 

 cavity. How the metamorphosis takes place in this and in 

 the other three cases (Ascetta mirabilis, Asculmis armata, and 

 Leuculrnis echinus) I cannot say in the present state of our 

 knowledge. 



After what has been said, I need hardly say particularly 

 that all the inferences founded by Hackel upon the " homology " 

 of the sponge-larvse (Gastrida) with the larvse of other animals, 

 collapse of themselves, because they are destitute of all solid 

 grounds. 



In conclusion, I will make one or two remarks upon the 

 question of Coelenterism, but without entering into any detailed 

 discussion, as I have elsewhere (in the concluding chapter of 

 my " Studien iiber die Entwicklung der Medusen und Sipho- 

 nophoren," appearing simultaneously with this paper*) treated 

 this question in detail. Here I will only endeavour to show 

 that the opinions expressed by Hackel are by no means capable 

 of shaking my theoryf as to homologies of the ccelenteric ap- 

 paratus, inasmuch as they for the most part rest upon miscon- 

 ceptions. Hackel's course of thought is as follows : — 1, " the 

 true body-cavity," which occurs only in the Vermes f, Echino- 

 dermata, Arthropoda, Mollusca, and Vertebrata, " always ori- 

 ginates by a cleaving of the mesoderm ;" 2, " as the mesoderm 

 is entirely wanting in the sponges, there can be no body-cavity 

 in them — nor does it occur in the Coelenterata;" 3, " the 

 true body-cavity can never, like the intestinal or stomachal 

 cavity, be surrounded by the entoderm ;" 4, " consequently 

 also the cavities of the gastro-canal-system in the Sponges and 

 Acalephs are not body-cavities, but an intestinal cavity " (p. 469). 

 To this I must object : — 1, the body-cavity in many animals 



* Zeitsch. fur wiss. Zool. Band xxiv. (1874) pp. 15-83. 



t This theory is that the gastrovascular apparatus of the Coelenterata 

 corresponds to the complex of organs which in the Echinodermata is formed 

 from the lateral diverticula of the primitive intestine. Consequently the 

 peritoneal cavity with the water-vascular system is to be regarded as the 

 homologue of the gastrovascular system. This theory is supported by a 

 whole series of facts, as is more particularly explained in my memoir just 

 cited. 



% It may here be mentioned in passing that the notions accepted by 

 Hackel of the Vermes accehmi and V. ccelomati by no means possess the 

 importance which that naturalist ascribes to them. The Nemertina and 

 Microstomea have a u true body-cavity " as well as several Trematoda, at 

 least in the states of redipe and sporocysts. 



