12 CRUSTACEA CHAP. 
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 the 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 
which metamerically repeated mesodermal cavities have been 
seen in the embryo Crustacean is that of Astacus; here Reichen- 
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 subsist 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 Malpighian 
tubes present in some Amphipods which open into the alimentary 
canal, and resemble the Malpighian tubes of Insects, consist of 
i Abhandl. Senckenberg. Nat. Gesellsch. xiv., 1886. 
