792 



CLASS V. ARACHNIDA. 



abuts against the epiblast cells of the surface at a point a little anterior 

 (dorsal) to the point of invaginatioii. Here the median eyes are formed. 

 The mode of their development has not been followed in detail, but it 

 appears to be similar to that of the median eyes of Scorpio. The 

 cells of the superficial epiblast (hypodermis) secrete the lens, and 

 become the " vitreous body." The underlying cells of the invagination, 

 the originally deep ends of which are now directed towards the surface, 

 form the retina. 



In the case of the lateral eyes no invagination leading to a reversal of 

 the retinal layer occurs. 



17 



Fio. 514. Limulus polyphemus. Diagrammatic view of the left half of the body, seen from 

 the inner side. 1-5 the second to the sixth cephalothoracic limbs (the first, left chelicera, 

 is hidden by the camerostome, above the base of 1 ; the seventh, the left chilariuin, is num- 

 bered 15) ; 6 the operculum ; 7-11 the second to the sixth abdominal limbs ; 12 mouth : 

 13 anus ; 14 caudal spine ; 15 chilarium ; 18 oesophagus : 17 proventriculus ; 18 intestine ; 

 19 the openings of two hepatic ducts ; 20 lobes of the liver ; 21 heart with eight venous 

 ostia : 22 branchial veins, returning the blood to the pericardium ; 23 anterior median 

 (frontal) artery ; 24 its point of division into two marginal arteries : 25 anterior paired 

 artery ; 26 supra-anal artery ; the subanal is seen below it. The central nervous system 

 is represented in black, surrounded by a blood sinus (from Shipley and MacBride, after 

 Leuckart, and, in part, after Packard). 



Alimentary Canal. From the mouth the oesophagus passes 

 forwards and dilates first into a proventriculus and then into a 

 somewhat globular muscular stomach, whose walls are raised 

 into ridges and covered with chitin. At the projecting pyloric 

 orifice (the limit of the stomodaeum) the chitin ends. The 

 intestine runs straight back to the short rectum, receiving in the 

 cephalothorax the orifices of the two pairs of hepatic ducts. 

 The Liver is of great size, and disposed in symmetrically arranged 

 masses, its acini together with those of the generative gland 

 forming a packing for the other organs of the cephalothorax. 

 It also extends back along the intestine into the abdomen. 



The Heart (Fig. 515) is fusiform, and lies dorsal to the intestine 

 over its anterior three quarters. It is surrounded by a peri- 

 cardial sinus, communicating with it by eight pairs of ostia. 

 It gives off an anterior median (frontal) and five paired arteries, 



