164 ERNST HAECKEL. 



The true body-cavity of animals, the coelom (or the so- 

 called •' pleuroperitoneal cavity"^) has, therefore, also arisen 

 phylogenetically in a similar manner by the two muscular 

 layers or middle germ-layers shrinking apart, as is known 

 to be the fact ontogenetically, since the time of Kemak, in 

 the embryology of Vertebrata. The mesentery is formed 

 Avhere the two layers remain in connection, and keep the in- 

 testine firmly attached to the body-wall, I have already 

 fully detailed my morphological idea of the body-cavity in 

 the 'Biology of Calcareous Sponges' (p. 467), and, therefore, 

 content myself here with again expressly making it promi- 

 nent that, according to my view, the coelom has first arisen 

 in the Vermes by the above-mentioned process (shrinking 

 apart of the two muscular layers), and has been transferred 

 as an inheritance from these to the four higher groups of ani- 

 mals. On the other hand, the coelom, or true body-cavity, 

 is absent in all the Zoophyta (sponges and Acalephse), as well 

 as in the lowest worms, the Plathelminthia (Turbellaria, 

 Trematoda, Cestoda). With the coelom, the blood, and 

 the vascular system is at the same time absent in these 

 animals, for these parts are inseparably united. Where the 

 first trace of true body-cavity appears, the first blood is 

 also already present, namely, the secretion which fills the 

 latter, the primitive " Hsemolymph" or " Hsemochyle." 



This view of the coelom places me in direct antagonism with 

 the view of Leuckart, already shared by most zoologists, who 

 attributes to the Zoophytes (his Ccelenterata) a true coelom, 

 and as late as 1869 represented his opinions thus: "The 

 body-cavity of the Ccelenterata does not lie between the 

 exoderm and entoderm, but is enclosed by the latter,^ just 



^ The teclmical term ccelom, which 1 have proposed in the 'Biology of 

 Calcareous Spouges' (p. 468J, lor the true body-cavity of auimals, is preler- 

 able to the hitherto customary expression " pleuroperitoneal cavity/' not only 

 on account of its greater shortness and convenience, but, before all, because 

 the other term is not applicable at all in a strict sense to the Invertebrata, 

 and applies to the latest and most differentiated condition of the coelom, as 

 it occurs only in the highest Vertebrata. 



2 Leuckart says (in the 'Archiv tiir Naturgeschichte,' 1870, II, p. 270), 

 "The opinion that the inner hollow structure of Ccelenterata corresponds iu 

 its morphological significance to the body-cavity of other animals, has met 

 with pretty general acceptance in our science — a view which anatomical 

 relations not only justify, but also compel the observer to agree with," &c. 

 In reply to this we may remark that Van der Hoeven characterised tlie 

 Acaleplise twenty years ago in his ' Natural History ' (furnishing corrections 

 to Leuckart) quite correctly in the following words : — " Ventriculus paren- 

 chymato corporis sine cuvttale abdominali mclusus : canales e veutriculo 

 ortura ducentes." Gengenbaur (1^61), Noschin (1865), Semper (1867), 

 and Kowalevsky (1S68J, have subsequently in the same sense correctly 



