338 THE CRAYFISH AND THE ARTHROPODA 



tern and are therefore called a hiemocoele. When, however, all the 

 facts of Comparative Anatomy and Embryology are brought 

 together, it seems probable that the cavity within the reproductive 

 organs, from the walls of which the germ cells originate, represents 

 the degenerate remains of a coelomic cavity; and that the repro- 

 ductive ducts, which may open on different segments in different 

 types of Crustacea, or on different somites in the two sexes, repre- 

 sent so many greatly modified nephridia. The excretory organs 

 suggest a similar relationship, since there are two types, the green 

 glands of the Malacostraca opening on the antennal segment, and 

 the shell glands of the Entomostraca opening on the somite that 

 bears the second pair of maxillae. The nervous system closely 

 resembles that of the annulate and is similarly related to the 

 digestive tract. The musculature is too specialized for any com- 

 parison with the simple arrangement in the body wall of the worm ; 

 as is also the circulatory system, with its unique feature of ostia 

 in the walls of the heart. In many Crustacea, however, the heart 

 is an elongated tubular organ and thus suggests the pulsatile dorsal 

 vessel of the segmented worms. The sternal artery, in such 

 animals as the crayfish, resembles the sub-neural -vessel of the 

 worm as it extends along the ventral side of the body; while its 

 vertical portion, which lies on one or the other side of the intestine 

 in different individuals, is perhaps a survival of the lateral con- 

 nections between dorsal and ventral vessels in the Annulata. 

 The crustacean plan of body may, therefore, be compared with 

 that of the Annulata, if we suppose that Crustacea are annulate- 

 like animals that have been greatly modified in the manner 

 indicated. 



The Phylum Arthropoda 



Other representatives of the great Phylum Arthropoda may now 

 be considered. No other phylum of the Animal Kingdom ap- 

 proaches the arthropods in the number of species included. There 

 are over 360,000 known species of insects alone, as compared with 

 a total of some 35,700 species of the Chordata and 60,000 of the 

 Mollusca, these two being the only other phyla containing many 

 thousands of species. If to these 360,000 insects be added 16,000 

 crustaceans, 16,000 arachnids, and 2,000 myriapods, the grand 

 total of 394,000 species is more than three times larger than that of 

 all other species of animals added together (128,400). We are 



