I 2 CRUSTACEA 



blood to the pericardium, and so to the heart again through the 

 apertures or ostia which pierce its walls. 



This condition of the body-cavity or haemocoel is reproduced 

 in the adults of all Arthropods, but in some of them by following 

 the development we can trace the steps by which the true coelom 

 is replaced by the haemocoel. In the embryos of all Arthropods 

 except tlie Crustacea, a true closed metamerically segmented 

 coelom is formed as a split in the mesodermal embryonic layer 

 of cells, distinct from the vascular system. During the course 

 of development the segmented coelomic spaces and their walls 

 give rise to the reproductive organs and to certain renal organs 

 in Peripatus, Myriapoda, and Arachnida (nephridia and coxal 

 glands), but the general body-cavity is formed as an extension 

 of the vascular system, which is laid down outside the coelom 

 by a canaliculisation of the extra-coelomic mesoderm. In the 

 embryos of the Crustacea, however, there is never at any time 

 a closed segmented coelom, and in this respect the Crustacea 

 differ from all other Arthropods. The only clear instance in 

 wliich metamerically repeated mesodermal cavities have been 

 seen in the embryo Crustacean is that of Astacus ; here Eeichen- 

 bach ^ states that in the abdomen segmental cavities are formed 

 which subsequently break down; but even in this instance no 

 connexion has been shown to siibsist between these embryonic 

 cavities and the reproductive and excretory organs of the adult. 



Since the connexion between the coelom and the excretory 

 organs is always a very close one throughout the animal 

 kingdom, interest naturally centres upon the renal organs in 

 Crustacea, and it has been suggested that these organs in 

 Crustacea represent the sole remains, with the possible exception 

 of the gonads, of the coelom. Since, at any rate, a part of the 

 kidneys appears to be developed as a closed sac in the mesoderm, 

 and since they possess a possible segmental value, this suggestion 

 is plausible ; but, on the other hand, since there are never more 

 than two pairs of kidneys, and since they are totally unconnected 

 with the gonads or with any other indication of a segmented 

 coelom, the suggestion remains purely hypothetical. 



The renal organs of the Crustacea, excluding the Malpio-hian 

 tubes present in some Amphipods which open into the alimentary 

 canal, and resemble the Malpighian tubes of Insects, consist of 



' Abhandl. Senckenherg. Xat. Gesellsch. xiv. , 1886. 



