322 ZOOLOGY. 



of the crayfish are of great interest. One is the complete 

 absence of ciliated epithelium ; the other is the high develop- 

 ment of the muscular tissue, all of which is striated. 



27. Classification. The crayfish is obviously allied to the 

 various marine lobsters, shrimps, and prawns, and less closely 

 to the crabs. These constitute the highest order of the class 

 Crustacea, called Decapoda from their possessing ten conspicu- 

 ous limbs, viz. the pair of pincers and four pairs of walking- 

 legs. The Decapoda, together with the wood-lice, sand- 

 hoppers, and less familiar forms, form a division of Crustacea 

 with a fixed number of segments, viz. 19; whereas the humbler 

 water- fleas, barnacles, etc., have no such constant number. 

 All these forms constitute the class Crustacea. Its nearest 

 allies are probably the king-crabs, scorpions and spiders 

 constituting the Aracknida ; and less nearly, the Centipedes 

 and Insects. The whole of these constitute the phylum 

 Arthropoda, characterized by the jointed appendages, 

 peculiar ostiate heart, absence of ciliated epithelium, and 

 certain other points. 



Comparing the crayfish with our other Invertebrate types, 

 we at once think of the similarity in the metamerism and 

 nervous system to the earthworm. But the wide differences 

 in the ccelom and vascular system, excretory and reproduc- 

 tive organs show how great a gap there is between them. 



Looking back on our Invertebrate types, the student will 

 realize that they do not form a homogeneous assemblage, 

 united by close relationship, as do the Vertebrates. In fact, 

 the historical division of the Animal Kingdom into Verte- 

 brata and Invertebrata can no longer be upheld as a natural 

 one. As we have seen, the primary division of Animals 

 must be into Protozoa and Metazoa. The Metazoa again 

 must first be divided into animals with two germinal layers, 

 or Coelenterata, and those with three germinal layers, 

 termed Ccelomata (not a very happy term, since not all of 

 them possess a coelom). After this no such sharp divisions 

 in classification can be made. While among Coelomata at 

 least five well-defined and important phyla can be recog- 

 nized viz. Echinodermata, Mollusca, Chsetopoda, Arthro- 

 poda, and Vertebrata no one of these, taken as a ivhole can 



