174 



SYSTEMATIC ZOOLOGY 



All these six pairs are regarded as biramous, thus being built 

 on the plan of the typical crustacean appendage and showing a 



relationship to that group. The ab- 

 dominal appendages bear, on their 

 posterior surfaces, numerous delicate 

 folds, which function as gills ; these 

 folds, arranged at right angles to the 

 surface of the appendage, resemble the 

 leaves of a book. At the posterior end 

 of the abdomen on the ventral side is 

 the anus. On the dorsal side of the 

 cephalothorax are two simple eyes, 

 symmetrically placed near the median 

 line, and two compound eyes much 

 farther apart. The sexes are separate 

 and the development is interesting, since a larva is formed, 

 resembling to some extent the adult, but without a tail and 



FlG. 167. Limulus. Embryo in 

 the trilobite stage; enlarged. 

 o, eye. (After Dohrn, from 

 Clans and Sedgwick's Text- 

 book.) 



A 



lbi 



^ ' 



Fig. 168. A, Dalmaniies socialls, dorsal aspect; B, the same rolled up, lateral aspect; 

 C, ventral side of head of Phacops fecundus . c.sh, cephalic shield; <-. eye : \. , fixed cheek; 

 f.s, frontal suture ; gl, glabella ; Ibv, labium ; m.c, movable cheek ; />, pygidium ; //, pleura : 

 s./.p, subfrontal plate; ///.thorax. (After Gerstaecker, from Parker and Haswell's Text- 

 book.) 



