236 ERNST HAECKEL. 



body of the Metiizoa ; but, simultaneously Avith its origin, 

 has proceeded the separating of the two cell-layers of its 

 wall, the inner nourishing epithelia (the gastral lamellae or 

 entoderm), and the outer investing epithelia (the dermal 

 lamellae or exoderm). 



In the second line of succession the elements of the 

 skeleton-system (in the majority of the Metazoa ?) formed 

 themselves, and this in the layer of the exoderm, as the 

 sponges teach us. Although in the sponges the two pri- 

 mordial germ-lamellae have (universally ?) remained constant 

 in their original simplicity, and no third germ-lamella has 

 developed itself from them, yet in the thickened exoderm of 

 many of them Ave find present a very complicated and ex- 

 tensively differentiated skeleton-system. Indeed, already the 

 Protozoa have very generally formed skeleton-parts both for 

 protection and support. It is unnecessary to mention in 

 addition that the skeleton-system in the different groups of 

 animals is of different epochs and of phylogenetic origin. 



In the third line of succession the nervous and muscular 

 systems develop themselves simultaneously. The beautiful 

 investigations of Kleinenberg on the ontogenesis of Hydra^ 

 have informed us of the simultaneous origin of these two 

 systems of organs which here exist in the most intimate 

 reciprocity. The highly interesting neuro-muscular system 

 of the Hydra is placed immediately before our eyes in statu 

 nascenti. The neuro-muscular cells developed from the 

 exoderm of the Hydra show us the functions of both still 

 united in a single individual of the first order. The two 

 systems of organs first appear independent and opposed to 

 each other, through a separation and a division of labour 

 into nerve- cells and muscle-cells. True muscles, in the 

 strictest sense of the term, therefore, occur first in those 

 animals in which true nerves also appear, and vice versa. 

 As the Acalephae show us, only the dermal or parietal neuro- 

 muscular system has originated at first from the outer germ- 

 lamellae. The gastral or visceral neuro-muscular system 

 (intestinal muscles and nerves) has probably originated in- 

 dependently in a perfectly analogous manner from the intes- 

 tinal glandular lamellae. Hitherto nothing has been said 

 against the view that the visceral nervous system has arisen 

 independently of the parietal ; the former is just as much in 

 connection with the intestinal muscular layer as the latter 

 with the dermal muscular layer. 



In the fourth line of succession the kidney or excretory 



' An account of thfse investigations is given by Prof. AUman in the 

 January number of this Journal. — Ed. 



