118 



UNDERWATER GUIDE TO MARINE LIFE 



SEA STRIDER. 

 HQloborHes--§-i»i. 



HORSESHOE CRAB 

 Limu.lixs- l&in. 



SEA SPIDE 

 Nymphon-3in. 



Fig. 39. Arachnids and insects. Adapted from the Picports of H.M.S. Challenger 

 (^1873—76^ and drawn from life. 



years ago, before the age of fishes. In Silurian times, 350 million years ago, 

 some scorpionlike arachnids became the first animals to venture onto land. 

 Now, marine arachnids are restricted to a few animals which are either hold- 

 overs from the past or have very specialized habits. 



The body of arachnids is sharply divided into a cephalothorax (head-body) 

 and an abdomen. There are no antennas and no true mouth parts that could 

 be called jaws, the mouth parts always being of the sucking variety (except for 

 the horseshoe crab). 



There are only two arachnid groups in the sea. The first of these contains 

 the living fossil, the horseshoe crab, Limuhis, which has existed on earth 

 virtually unchanged for almost 200 million years. It is found on sandy shores 

 over much of the earth, especially in the temperate seas of North America and 

 in the western Pacific. Horseshoe crabs are harmless, but the feet of swimmers 

 have accidentally been pierced by the long tail spine. Very large horseshoe crabs 

 may be over IV2 feet long. They crawl slowly over the bottom, grasping in- 

 vertebrates with their clawed feet. Prey is "chewed" by means of the hard 

 bases of the legs near the mouth. They burrow in soft bottoms and can swim 

 weakly by paddling with their feet. Under the abdomen of the horseshoe 

 crab are a series of leaflike plates. These are the book gills which furnish these 

 animals with oxygen from the water. Commensal flatworms are often found on 

 the leaves of these gills. 



The sea spiders or pycnogonids are among the most incredible animals found 

 in nature. They are very often of red coloration since several species li\c in 

 rather deep water. They range in size from less than an inch in leg span to 

 over 4 or 5 inches. The body is small, and the abdomen almost nonexistent, 

 but the legs are extremely long and contain branches of the stomach which 

 ordinarily would be in the abdomen if the abdomen were large enough. The 

 mouth parts are of a sucking type; sea spiders live by sucking the juices from 



