276 ZOOLOGY. 



portion with one or more segments, supporting two jointed 

 branches, a median and an external. Appendages may be 

 entirely wanting (as in the abdominal segments of insects) ; 

 and yet these may appear in a rudimentary form in the early 

 stages of the embryo, only to disappear later. 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 some- 

 times aborted in regions where they are no longer needed. 

 General groups of appendages are as follows: (i) pre- 

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

 ing structures, mandible and maxillae; (3) thoracic, chiefly 

 walking appendages; (4) abdominal, variously modified (as 

 swimmerets, gills, etc.) or wanting. The wings are not to be 

 regarded 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 protec- 

 tion for the second pair. Either pair, more often the second, 

 may be aborted. 



310. Ccelom. The development of the arthropods shows that the 

 spaces in the body are not truly ccelomic 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. 



311. 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 connective tissue, the dermis, containing nerves and blood 

 vessels. Still within these are the longitudinal muscles of the 

 body wall. When the secreted shell becomes thick and hard, 

 further growth is necessarily more difficult. This difficulty is 

 usually overcome by moulting, in which process the old 



