420 COMPARATIVE ANATOMY CHAP. 



in the hypodermis. The compound eye of Limulus is thus seen to be 

 composed of numerous independent unilaminate single eyes crowded 

 together. Each single eye corresponds with the unilaminate eye of 

 certain Myriapoda and Scorpionidce, and the whole compound eye cor- 

 responds with the sum of the eyes on one side of these Arthropoda, 

 except that the chitinous carapace in Limulus forms a thickening 

 common to all the single eyes. 



Enteric Canal. From the large mouth a long oesophagus rises 

 upward and forward, to enter a muscular masticatory- or fore-stomach 

 placed in the anterior part of the cephalothorax ; the chitinous intima 

 of this stomach projects in numerous longitudinal folds into its lumen. 

 The fore-stomach is followed by a long straight mid-gut widened at its 

 commencement ; this runs through the cephalothorax and abdomen and 

 opens externally at the base of the caudal spine through a ventral anal 

 aperture of the short hind-gut. The mid-gut receives the 4 ducts of 2 

 pairs of hepatopancreatic glands which branch freely in the cephalo- 

 thorax. All through the intestine, except in the mid-gut, a chitinous 

 intima is found. 



Circulatory System. The heart is an elongated dorsal vessel 

 provided with 8 pairs of ostia which can be closed by valves. 



Sexual Organs. The sexes are separate. The male, which is 

 smaller than the female, is further distinguished externally by the fact 

 that the most anterior or the two anterior postoral pairs of limbs do 

 not end in pincers, but in claws. 



The 2 ovaries are tubes forming a network of branches, thos of 

 the two sides communicating with each other at various points. The 

 two oviducts form a sac-like wider portion before they emerge. The 

 female sexual apertures lie on the inner side of the opercular plates (the 

 side turned to the body), at their bases, to the right and left of the 

 median line. The two testes consist of a large number of vesicles 

 dispersed throughout the body and attached to sperm ducts which 

 branch and anastomose freely. The male apertures have the same 

 position as the female. 



Coxal Glands. On each side of the cephalothorax lies a red gland 

 of considerable size whose outer aperture has only been found in 

 young animals on the basal joint of the fifth pair of limbs. It is un- 

 certain whether these coxal glands correspond with the shell glands of 

 the Crustacea (which also emerge on the 5th pair of extremities, i.e. on 

 the 2d maxillae). We have no right to assume that the 5th pair of 

 extremities of Limulus answers to the 2d pair of maxillae of the 

 Crustacea ; it is indeed improbable that this is the case. 



Ontogeny. The 6 anterior pairs of limbs appear first and simultaneously, then 

 follow the 7th pair (operculum) and the 8th (first gill-carrying abdominal limbs). 

 On the cephalothorax there are indications of segmentation. The young Limulus 

 hatched from the eggs shows a complete rudimentary cephalothoracic shield, the 

 segmentation having then entirely disappeared. The abdomen, on the contrary, 

 appears distinctly formed of 8 segments, but these are not movable upon each other. 



