Introductio7i to Anifnal Alorphology. 381 



There are three orders : — 



Order i. Chilognatha {Latreille) — cylindrical, or semi- 

 cylindrical, with two pair of legs on every segment after the 

 fifth or sixth ; thoracic rings with free dorsal plates ; each 

 segment has a stigma in front of the articulation of each leg ; 

 often very small, except in Polydesmus ; the genital orifice is 

 between the second and third ring, the latter of which has no 

 legs ; the small antennae consist of seven joints, the last 

 often hidden in the sixth ; the coxopodites of the first pair of 

 legs are directed forwards, and partly unite to form a labium. 

 In development new segments arise between the third last 

 and second last segment of the hexapod larva. The number 

 of somites is not always specifically constant, and the shell 

 of each joint is either a ring or half ring, in Glomeris allow- 

 ing the body to be rolled into a ball ; sometimes the ventral 

 plates are divided, and extended laterally as laminae pleurales. 

 The first thoracic ring has no limb in Polydesmus, and when 

 the seventh ring bears the penis it has no limb. In lulus the 

 two testes are united by transverse anastomoses ; on each 

 side of the back on all (lulus, Polyzonium), or some seg- 

 ments (Polydesmus), 2iX& foranmia repugnatoria. 



Three families are included: — i. Glomeridae — wood- 

 louse-like, short, concave below, semicylindrical, with either 

 twelve segments and seventeen pair of legs (Glomeris), or 

 thirteen segments and twenty pair of legs (Sphaerotherium) ; 

 each ring has one dorsal and four ventral plates, the lateral 

 of which (laminee pleurales) articulate with the legs ; the first 

 ring is small, covered by the second, the last is shield-like ; 

 the spicula are pre-anal, placed far back. 2. lulidae — cylin- 

 drical ; ventral and dorsal plates united in a suture ; head large, 

 free ; first thoracic ring not covered by the second ; last ring 

 compressed, with an anal slit. The eyeless Polydesmus and 

 Strongylosoma have twenty rings ; Polyxenus has nine. They 

 have also a sternal space between the origin of the legs, and 

 a lateral plate, or ridge, or bundle of hairs. lulus has fifty, 

 Lysiopetalum forty-five rings, and the limb-pairs medially in 

 contact. Spirobolus reaches a length of nine inches. 3. Si- 

 phonizantia — bodies semi-cylindrical, capable of being spi- 

 rally rolled up ; head small, hidden under the first body ring ; 



