RELATIONS OF INSECTS TO OTHER ARTHROPODA 



more like feet than jaws, while they have in most respects a similar 

 mode of embryonic development, the larval forms being also similar. 



The Merostomata. The only living form, Limulus, is undoubtedly 

 a very primitive type, as the genital glands and ducts are double, 

 opening wide apart on the basal pair of abdominal legs (Fig. 3). 

 Moreover, their head-appendages, which are single, with spines on 

 the basal joint, are very primitive and morphologically nearer in 

 shape to those of the 

 worms (Syllidse, etc.) 

 than even those of the 

 Crustacea. Besides, 

 their four pairs of coxal 

 glands, with an external 

 opening at the base of 

 the fifth pair of head- 

 appendages, and which 

 probably are modified 

 nephridia (Crustacea 

 having but a single pair 

 in any one form, either 

 opening out on the 

 second autennal, green 

 gland, or second maxil- 

 lary, shell-gland, seg- 

 ment), indicate a closer 

 approximation to the 

 polynephrous worms. 

 Limulus has other ar- 

 chaic features, espe- 

 cially as regards the 

 structure of the simple 

 and compound eyes and 

 the simple nature of the 

 brain. 



The Trilobita. These 

 archaic forms are still 

 more generalized and primitive than the Merostomata and Crus- 

 tacea, and probably were the first Arthropoda to be evolved from 

 some unknown annelid worm. They had jointed biramose limbs of 

 nearly uniform shape and size on each segment of the body, which 

 were not, as in Crustacea, differentiated into antennae, jaws (man- 

 dibles), maxillae, rnaxillipeds, and two kinds of legs (thoracic and 



FIG. 1. Restoration of under side of a trilobite (Triar- 

 thrux becff.i), the trunk limbs bearing small triangular respira- 

 tory lobes or gills. After Beecher. 



