ARTHROPODA OF DOUBTFUL POSITION. 



83 



individuals, and in variety of forms, the order attained its liighcst point in the upper 

 Silurian. About four hundred species distributed among tifty genera have been de- 

 scribed. 



Of their habits nothing definite is known. Dr. Burmeister thought that they lived 

 in large schools in the shallow water along the shores, swimming back downwards like 

 so many Phyllopoda to-day. Dr. Packard, on the other hand, thinks that, like the 

 living horseshoe crab, they burrowed just beneath the surface of the mud and sand at 

 the bottom of shallow waters, — a view which we are inclined to consider most 

 probable. 



Order II. — MEROSTOMATA. 



The genus Ziimulus, which embraces the forms familiarly known as horseshoe 

 crabs, king-crabs, and Molucca-crabs, is the only living representative of the group 

 Gigantostraca. The livhig species have 

 a peculiar distribution. On our own 

 coast Limulus polyphemus extends 

 from Maine to Florida and the West 

 Indies, while L. moluccamis and ro- 

 tundicaxulus and two other doubtful 

 species are found on the eastern shores 

 of Asia. No examples are found on 

 the western shores of either continent, 

 and none have been reported from 

 South America. 



The horseshoe is composed of three 

 portions movably articulated to each 

 other. The anterior, the cejjhalothorax, 

 closely resembles in outline the foot of 

 a horse, and gives the common name 

 to these animals. Behind this comes a 

 wedge-shaped abdomen, and then a long, 

 bayonet-shaped caudal spine. On the 

 upper surface of the cephalothorax are 

 the eyes. Of these two are compound, 

 one on either side of the body, while 

 near the front margin in the median 

 line a couple of simple eyes occur. The 

 lower surface of the cephalothorax 

 closely follows the upper, so that in- 

 stead of the thick body which would be 

 expected from seeing the vaulted back, 

 the body is really very thin. Near the 

 centre of the lower surface arise six 

 pairs of legs, and between the second 

 pair is the mouth. The first pair are 



small, and differ considerably in the two sexes ; in the female they end in a regular 

 pincer, but in the male the finger is bent and folds over the thumb. The next four 

 pairs of feet may either terminate in a pincei-, or one or more may end in a simple 



-Under-surface of liorseshoe crab, Limulva poltj- 

 Abdomen. r. Cephalothorax. r/. The tirst 

 gill-bearing abdominal appendage. »!. Mouth, o. Oper- 

 culum, s. Metastoma. 1 to 6. Cephalothoracic limbs. 



phemus. 



