232 



MANUAL OF ZOOLOGY 



shgl 



(Fig. 132) the abdomen with its appendages is imperfectly 

 developed, and not enclosed completely in a hard exoskele- 

 ton, this region being sheltered in the shell of a whelk 



or other univalve mollusc 

 which the hermit-crab drags 

 about with it. 1 



The crustaceans enumer- 

 ated above, together with 

 the sand-hoppers, woodlice 

 and their allies, and a large 

 number of others, form one 

 or two sub-classes into which 

 the class Crustacea is di- 

 vided — the sub-class Mala- 

 costraca. The Malacostraca 

 are highly organised Crus- 

 tacea, usually of consider- 

 able size, and nearly all 

 have a thorax of eight and 

 an abdomen of seven seg- 

 ments. The appendages 

 are highly differentiated. 

 There is a gastric mill, and 

 the renal organs are in the 

 form of antennary glands. 



The other sub-class is 

 the Entomostraca. The 

 Entomostraca, which are 

 even more numerous than 

 the Malacostraca, are of comparatively simple organisation, 

 and usually of small, often almost microscopic, size. The 



1 Our common American hermit-crab is Eupagurus pollicaris ; the 

 right chela is still larger than in Pagurus bernhardus. of the European coast. 



133. — Apus glacialis, ventral aspect. 

 abd.f, abdominal feet; ant. 1, anten- 

 nule; ant. 2, antenna; Ibr, labrum; titei, 

 mandible; mx, first maxilla; ov, aper- 

 ture of oviduct; s f pi, sub-frontal 

 plate; sk.gl, shell-gland; th.f, thoracic 

 feet; th.f. i, first thoracic foot. (After 

 Bernard.) 



