28o ZOOLOGY 



Where the metamerism is obscured by fusion, the number of 

 appendages may be the only indication we have of the number 

 of segments; but as we have seen, the appendages themselves 

 are sometimes aborted in regions where they are no longer 

 needed. So it is not always possible to determine how many 

 segments are really represented in an animal. 



General groups of appendages are as foUows: (i) preoral, 

 mostly sensory, — as antennas; (2) oral, biting and sucking 

 structures, — mandible and maxillas; (3) thoracic, chiefly walk- 

 ing appendages; (4) abdominal, variously modified (as swim- 

 merets, gills, etc.), or wanting. The wings are not to be re- 

 garded as homologous with the jointed appendages. They 

 originate as expansions of the integument of the body, sup- 

 ported by numerous tubular "veins" containing blood spaces, 

 tracheas, and nerves. Wings, when present, comprise one (flies) 

 or two pairs (bees). Often the anterior pair is hardened and 

 serves merely as a protection for the second pair. Either pair, 

 more often the second, may be aborted. 



317. Coelom. — The development of the arthropods shows that the spaces in 

 the body are not truly ooelomic as a rule, but are, so to speak, much enlarged 

 blood spaces containing the corpuscle-bearing fluid. The pericardial sinus is 

 one of these. Such a body cavity is known as a hcemoccele. 



318. Integumentary Structures. — The arthropod integument 

 has a hypodermal layer of cells which secretes the chitinous 

 cuticle constituting the external skeleton. The chitin may be 

 mixed with salts of lime. Beneath the hypodermis, in certain 

 groups, is a layer of connective tissue, — the dermis, containing 

 nerves and blood vessels — within which are the longitudinal 

 muscles of the body wall. In Insecta, Arachnida, and Myria- 

 poda the body wall is composed of a cuticula, a hypodermis and 

 a basement membrane. The chitinous covering serves for pro- 

 tection and support of the soft parts, and for the attachment of 

 the muscles of locomotion. At intervals this exoskeleton is 

 shed off (moulted), in which process the old cuticle is separated 

 from the hypodermis, rupturing along some line of weakness, 

 and allowing the escape of the animal. This moulting extends 

 not only to the minutest of the external organs, but to the stomo- 

 daeum and proctodaeum and certain other internal structures 



