ARTHROPOD A 279 



may be the only indication we have of the number of segments; 

 but as we have seen, the appendages themselves are some- 

 times aborted in regions where they are no longer needed. 

 So it is not always possible to determine how ma^Sy segments 

 are really represented in an animal. 



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

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

 structures, mandible and maxillae; (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 ribs or "veins" containing branches 

 of the blood vessels, tracheae, 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 Ccelom. The development of the arthropods shows that the spaces in 

 the body are not truly coelomic 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 hamoccele. 



318. Integumentary Structures. The arthropod skin has 

 an epidermal layer of cells which secretes the chitinous cuticle 

 constituting the external skeleton. The chitin may be mixed 

 with salts of lime. Beneath the epidermis is a layer of con- 

 nective tissue, the dermis, containing nerves and blood 

 vessels. Still within these are the longitudinal muscles of the 

 body wall. This chitinous covering serves for protection 

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

 muscles of locomotion. When the secreted shell becomes thick 

 and hard, further growth is necessarily more difficult. This diffi- 

 culty is usually overcome by moulting, in which process the old 

 cuticle is separated from the epidermis, 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 stomodaeum and prqctodaeum as well. A 

 new cuticle begins to be secreted at once, but this "soft-shelled" 



