ARTHROPODA. 491 



connected to one another, either by a soft intersegmental membrane or by 

 fusion. The somite itself may be ring-like, or consist of a dorsal plate, the 

 tergum, and a ventral plate, the sternum, connected laterally by a soft 

 pleural membrane, or more rarely by distinct pieces, a dorso-lateral epi- 

 meron and ventro-lateral episternum. These last named parts may some- 

 times be distinguished as regions when the parts of a somite are con- 

 tinuous, e.g. higher Crustacea. The tergum may occasionally be broken 

 up secondarily, as in the thoracic somites of Insecta. 



A head region is nearly always distinguishable. It consists of a prae- 

 oral or pro-cephalic region, to which are fused a variable number of post- 

 oral somites, and it then either remains distinct (Insecta, Myriapodd} or 

 becomes continuous with a part or the whole of the thorax, forming a 

 cephalo-thorax (Arachnida, many Crustacea}. A thorax is not marked off 

 in the Myriapoda. In Insecta it consists of three somites ; in Arachnida it 

 may be considered as consisting of four ; in the higher Crustacea of eight, 

 and of a variable number in the lower. The somites behind the thorax 

 constitute the abdomen. This region is not specially differentiated in 

 Myriapoda, but is clearly distinguishable in the other classes. The number 

 of its somites varies much from class to class, and within the limits of the 

 same class. 



The Insecta, Myriapoda, and Protracheata carry upon the head a pair 

 of sensory antennae, which are probably to be regarded as processes of the 

 procephalic lobes, and not as homologous with the remaining appendages. 

 The Arachnida possess a pair of prae-oral appendages in the adult, which 

 are post-oral in the embryo. The first antenna of the Crustacea is probably 

 post-oral, like the second antenna and remaining appendages. The ap- 

 pendage or limb is composed of a series of articulated joints, and in the 

 Criistacea possesses typically a basal portion bearing an external and 

 internal branch (exo- and endo-podite). The muscles moving the joints 

 are contained partly within the limb itself, partly within the somite that 

 carries it. One pair of limbs at least, and usually more, are modified to act 

 as jaws. The external shape of the limb varies much, according to its 

 function. 



The body wall is composed of a chitinous cuticle, sometimes thin, 

 sometimes thick and then laminated, and in Crustacea hardened by cal- 

 careous deposit. It is formed by an underlying and single layer of 

 ectoderm or hypodermis cells. When the animal grows, this cuticle is 

 shed or undergoes ecdysis at stated periods, and at the same time its 

 internal extensions into the stomodaeum, proctodaeum, gland ducts, 

 tracheae, and sometimes the tendons, are shed also. Beneath the hypo- 

 dermis a thin basement membrane is nearly always to be detected. 

 Peripatus ( = Protracheata) alone has a continuous layer of circular muscles 

 and bands of longitudinal muscles, all of which are non-striated. In 



