SPIDER-CRABS AND KING-CRABS. 



539 



pointing obliquely backwards. Beyond this is a sharp horny spine not 

 jointed, and at least 

 as long as the rest 

 of the body. The 

 antennae, jaws, and 

 gill-feet all resemble 

 legs, and all termin- 

 ate in pincers except 

 the last pair. Two 

 compound and two 

 simple eyes are pre- Figt IS.-RING-CRAB (Limulus moluccanus). Reduced, 



sent. The legs are 



short, and are entirely covered by the shields. The few living species known 

 are found in the East and West Indies, and on the East coast of North America, 

 living in shallow water, or running over and burrowing in the sand. The species 

 figured, Limidus moluccanus (Clusius), has larger spines than the others. The 

 Limulia sometimes attain a length of two or three feet. They are the last 

 survivors of a great group of Crustaceans, now only imperfectly known 

 by their fossil remains. Three other orders, now wholly extinct, are referred 

 to the Gigantostraca: the Merostomata, the Hemiaspida and the Trilobita. 

 Some authors have recently proposed to regard Limulus as more nearly 

 allied to the Arachnida than to the Crustacea ; but its external resemblance 

 to the latter is certainly far more obvious. 



SUB-CLASS IV. PYCNOGONIDA. 



THESE are sluggish animals, found among sea- 

 weeds on the sea-shore, and somewhat intermediate 

 between Arachnida and Crustacea. The front of 

 the body consists of four well-marked segments, 

 the first of which is formed of three fused together, 

 each bearing a pair of long, jointed legs, and 

 the abdomen is very small. They are provided 

 with a long suctorial proboscis. In their young 

 state, they are parasitic on Hydractinice, and are pro- 

 vided with three additional pairs of limbs, which was 

 short or wanting in the adults. 



Fig. 14. Pycnogonum 

 morale, Muller. Nat. size. 



CLASS 2L ARACHNIDA (SPIDERS, SCORPIONS, 

 AND MITES}. 



THIS rather extensive group of animals is generally recognisable by a few 

 salient characters. The head and thorax, instead of being separated, as in 

 insects, are usually fused together into a single mass, called the cephalo- 

 thorax ; there are two pairs of jaws, one pair of which are sometimes 

 regarded as modified antennae ; from two to twelve simple eyes, variously 

 arranged in different species ; and usually eight pairs of legs, all situated on 



