MYRIAPODA. 515 



followed by a large ' basilar segment,' usually stated to be composed of 

 three or four fused somites 1 . The remaining somites are either all 

 similar, or some are smaller than others and partially concealed by them, 

 and each somite consists of a single tergal and sternal plate united at 

 the sides by a soft pleural membrane. In Diplopoda the tergal region 

 makes frequently (e. g. lulidae) at least three-fourths of a circle, or when 

 flattened dorso-ventrally it has lateral expansions (e. g. Polydesmidae). To 

 each tergum corresponds a pair of sterna, one in front of the other, and 

 the first somite has the tergum expanded anteriorly and it may then 

 conceal the head completely (Enpauropus] or partially as in many others. 



The antennae are 7-jointed in Diplopoda, and sometimes lodged in 

 grooves of the head. They are long and many jointed in Chilopoda. In 

 Pauropoda (Diplopoda} the last joint of the antennae is bifid and the organ 

 is terminated by three long jointed appendages. The mandibles have a 

 broad masticatory surface in the vegetable-feeding Diplopoda, a toothed 

 margin in the carnivorous Chilopoda, in some of which a rudimentary palp 

 is said to exist. The maxillae in the former group are represented by a 

 four-lobed plate derived from a single pair of limbs in the embryo ; in the 

 latter each maxilla consists of a palp and bilobed median process. The 

 Polyzonidae (Diplopoda) have the jaws united to form a sucking tube. 

 The first pair of post-maxillary appendages in Chilopoda is limb-like, the 

 bases of the limbs in contact, as is the case in the second pair, the limbs 

 of the ' basilar segment,' which form the stout curved poison claws. The 

 poison gland is lodged in the two last joints and opens on the convex side 

 of the apex. The third pair of limbs (which have no corresponding tergum) 

 is leg-like and sometimes wanting (Lithobius, Scutigera). The remaining 

 somites bear a single pair of jointed limbs, the last pair being usually 

 long and turned backwards. The first two or three somites in Diplopoda 

 may carry a single pair of limbs, of which the first pair are shorter and 

 turned forwards and sometimes modified in the male ; or there are two pairs 

 to every somite in Enpauropus, as there are in the middle and posterior 

 somites of other members of the class. The third somite is apodous in 

 lulidae, and the seventh in the male carries one pair of limbs and the copu- 

 latory organ. The limb is usually 7-jointed, the terminal joint being a claw. 



The chitinoid cuticle is generally thick and firm in the terga and 

 sterna. Hairs are sometimes well developed in Diplopoda, e. g. Polyxenidae, 

 in which they are aggregated in bundles. Bundles of spines occur in the 

 extinct Protosyngnatha, and in the extinct Archipolypoda six longitudinal 

 rows of spines or tubercles. The majority of Diplopoda possess a longi- 

 tudinal lateral series of foramina repugnatoria, the apertures of glands 

 secreting a mal-odorous fluid. In Fontaria sp. ? (Diplopoda) the secretion 

 contains a ferment and a substance which breaks up into hydrogen 



1 Cf. Balfour, Comp. Embryology, p. 325. 

 L 1 2 



