152 KRNST HAECKEL. 



obtained for the accurate division of the animal kinfrdom into 

 the two primary sections of Protozoa and Metazoa, between 

 which the Gastraea stands as a fast boundary-stone, by my 

 proving the existence of a primordial rudiment of an intes- 

 tine in the sponges, and my pointing out the development 

 from it of the two primary germ-lamelhp, which furnish the 

 same common basis for the original formation of the body, in 

 all Metazoa up to the Vertebratoe. On the other hand, the 

 demand arose to obtain for this firm boundary line a decisive 

 negative security by internal evidence ; that it should be 

 proved that all the Protozoa were totally without both a 

 rudimentary intestine and the two primary germ-lamellae. 

 In this respect the Infusoria only, especially the Ciliata, 

 presented considerable difficulties, as their position, till very 

 recently, has wavered backwards and forwards between the 

 primitive animals, the Zoophyta, and the worms. I hope 

 that through my lately published investigations into the 

 morphology of the Infusoria^ I have definitely decided this 

 difficult question, and also have thoroughly repelled the 

 attacks which have recently been made on the view first put 

 forward by Siebold (1845), that the Infusoria are unicellular 

 organisms, and therefore true Protozoa. 



The celebrated researches into the ontogenesis of several of 

 the lower animals, which A. Kowalevsky has published 

 during the last seven years (in the Memoirs of St. Peters- 

 burg Academy), and which I must consider the most important 

 and suggestive of all recent ontogenetic works,^ were of espe- 

 cially high value to me in proving the true homologies of 

 the two primary germ-lamellse, in all the Metazoa, without 

 M'hich the Gastraea-theory cannot be maintained. However 

 Kowalevsky does not accede to the homology asserted by 

 me to be complete, of the two primary germ-lamellae in the 

 diflerent groups of animals, and considers, for instance, 

 that the intestinal glandular layer of Insecta, and the 

 entoderm of the Hydroida, &c., are special structures. 

 He also differs from me considerably about the significance 

 of the secondary germ-lamellae. But, on the whole, I 

 think I may assert, that the important facts which he has 



1 ' Jenaisclie Zeitschrift,' vol. vii, 1873, p. 516, tt. 27, 28. 



' The ontogenetic works of Kowalevsky, especially those on Amphioxus, 

 Ascklia, Euaxes, Holothuria, &c., have not found by far the estimate which 

 they deserved. This misfortune is, perhaps, chiefly due to his extremely 

 careless and unmethodical method of description. It is very dilBcult to 

 understand him, not only because his train of tliought is deficient in logical 

 sequence and consecutive arrangement, but because the illustrations are 

 partly not described at all, partly wrongly numbered, or given without 

 sufficient reference to the text. 



