VIII 



AETHEOPODA 



237 



of their cells to form retinulae, they do not become subdivided into 

 smaller eyes (Fig. 187, oc.l.), each remains as an undivided compound 

 eye throughout life, and consists throughout of a single layer of cells ; 

 there is no vitelligenous layer. The lateral eyes are developed from 

 the sides of the cephalic lobe, and as growth proceeds the cephalic 

 lobe extends backwards along the sides of the prosoma, so that the 

 eye appears as if it belonged to the fourth segment. 



The appendages of the abdomen appear as plates, each with an 

 inner lobe which we may regard as the endopodite ; the first pair of 

 appendages becomes, however, progressively reduced in size, and the 



Fig. 187. — Ventral view of an embryo of Limulus longispina, 21 days old. 

 (After Kishinouye.) 



Sri, appendage bearing first gill book ; ir^, appendage bearing second gill book ; ch, clielieera; 

 chU, chilaria ; c.coe, cephalic coelom ; coe, coelomic sacs ; /aSi-5, the rudiments of five pairs of iiabella, 

 i.e. of exopodites ; U-P, the rudiments of the five pairs of walking legs ; l.h, lateral hump ; oc, 

 median eye ; oc.l, lateral eye ; op, genital operculum ; proof, proctodaeum ; sfom, stomodaeum. 



segment to which they belong ceases to be distinguishable. Eemnants 

 of° these appendages remain as "chilaria" forming an underlip. 

 The second appendage joins its fellow to form the genital operculum, 

 the rest form gill books. 



The embryo is hatched out as the so-called " Trilobite larva " 

 which gradually changes into the adult. In this larva the append- 

 ages have all attained their adult form, but the abdomen is still 

 distinctly divided into segments, and each segment, seen from above, 

 shows a median tergum, and, on each side, a horizontally extended 

 pleuron, separated from it by a groove like the body segment of a 

 Trilobite seen from above. In the region of the prosoma we have 



