352 Introductio7i to Animal Morphology. 



nerve cord forks posteriorly ; there is a trigonal^ 

 bayonet-shaped tail or telson ; the mouth has a small 

 labrum and a rudimental metastoma ; the hinder 

 shield or operculum is hexagonal ; the natatory feet 

 and tail are absent in the larva. The ambulatory 

 legs consist of — ist, a modified pair of antennae (the 

 front, short pair) ; 2nd, a pair of mandibles; 3rd and 

 4th, two pair of maxilla ; 5th and 6th, two pair of 

 maxillipedes. The third pair of thoracic limbs are 

 expanded, and united medially, forming a flat plate 

 over the five pair of swimming feet. The operculum 

 corresponds to the post-abdomen of the trilobite, and 

 has articulated to it six pair of marginal spines (post- 

 abdominal feet), while the tail is the appendix of the 

 last joint of the post-abdomen. The gills consist of 

 130-150 plates, like the leaves of a book, arising from 

 the outer part of the coxopodite of each limb, which 

 has, at its inner side, two lamellar terminal joints. 

 The male has a small cylindrical penis. They are 

 natives of the Moluccas and of N. America, and form, 

 one genus, Limulus. 



Perhaps to this group belong the palaeozoic fossils, 

 Pterygotus and Eurypterus, which have many free thoracico- 

 abdominal segments ; eyes as in Limulus ; mouth with a 

 broad metastoma ; antennae one pair chelate ; one pair of 

 broad maxillipedes serrated at their bases ; free abdominal 

 somites, with no appendages. 



Sub-class 7. Podophthalmia* {Leach) — heterono- 

 mously segmented Crustaceans, with twenty somites,t 



* This and the succeeding sub-class are kno\vn as Malacostraca 

 {Latreille), to distinguish them from the foregoing or Entomostraca ; they 

 have always thirteen thoracico-abdominal somites. 



t Twenty-one, according to those who reckon the telson as a 

 somite. 



