234 ERNST HAECK^L. 



the epidermis), do not come under consideration here 

 at all. 



The coelom or the body-cavity, the original " pleuro-peri- 

 toneal cavity," which is entirely absent in the Protozoa, Zoo- 

 phyta and Aca^lomi (Plathelminthes), is certainly homolo- 

 gous in the Coelomata, and in the four higher stem-groups of 

 animals. It originates everywhere as a slit between the two 

 muscular layers, and has apparently descended from the 

 Coelomati, the worms Avith blood, to the four higher groups 

 of animals. However, this homology is not to be established 

 by comparison with the cavity of segmentation, from which 

 Kowalevsky makes the coelom proceed (comp. above, p. 165). 

 The coelom is originally filled with a fluid, which, on account 

 of its varying characters, can be defined as haemolymph or 

 hajmochyle. But in the higher worms this nutritive fluid is 

 already diff"erentiated into two different constituents, into the 

 colourless chyle or lymph which fills the body- cavity, and 

 into the coloured blood, which circulates in the closed vas- 

 cular system. This difl'erentiation also recurs in the 

 Vertebrata. 



The intestinal muscular layer, or the intestinal fibruus layer 

 (the " vascular layer" of Baer, the intestinal fibrous layer 

 and middle layer of E,emak), appears to be entirely absent 

 in part of the class Zoophyta (in the sponges and the lowest 

 Acalephaj), and to develop itself in a peculiar form in another 

 part (in the higher Acalephse). 



In the Acoelomi it already begins to shape itself out as 

 the "intestinal muscular sheath," and has descended from 

 these to the higher Avorms (the Coelomati), and from the latter 

 to the four higher stem-groups of animals. There is nothing 

 in the way of our recognising a universal homology in this 

 within these five groups of animals (the Bilateria). 



The vascular system, which, as a whole, has developed 

 itself in connection with tlie coelom, is, therefore, also to be 

 compared within the five higher stem-groups of animals ; but 

 the question as to how far its separate parts, and especially the 

 heart, are homologous, is very difficult to decide. Accord- 

 ing to the sharp-sighted comparison of Gegenbaur, the 

 heart of the Arthropoda and Mollusca is originally homolo- 

 gous to a section of the dorsal main vascular stem of the 

 Vermes, while the heart of the Ascidia and Vertebrata is 

 homologous to a section of the ventral stem. 



The intestinal glandular layer, which remains constant as 

 the epithelial outer covering of the intestinal canal and its 

 o-landular appendages, is certainly, throughout the whole 

 animal kingdom (only excepting the Protozoa), from the 



