420 



HISTOillCAL GEOLOGY. 



3. Arachnids. 



Having the body in two parts, cephalotliorax and abdomen (but in the lowest, 

 Mites, only one, — the abdomen and thorax not separate segments) : as Spiders, Scorpions, 

 Mites, Ticks. 



4. Limuloids. 



Limuloids are a nearly extinct tribe of species, related more nearly to the Arachnids 

 than to Crustaceans. The only species in American waters is the Limulus polyphemus, 

 or Horse-shoe, common on the coast of southern New England and to the southward. 

 Limuloids differ from Crustaceans in not passing through the Nauplius stage in embryo- 

 logical development ; in having no antennae corresponding to the first pair in Crustaceans ; 

 and in having the two antennae of the second pair chelate ; that is, terminating in pincers, 

 and used for conveying food to the mouth, — a degenerate service for sense-organs. 



A Paleozoic group, under the ti'ibe of Limuloids, includes the Euryptcrids — aquatic 

 species having the long, jointed body of a Caligus among Crustaceans, but occasionally 

 several feet in length. For figures, see pages 556, 623. They have two antennse, like the 

 Limulus, or none, and, moreover, the basal joints of part or all of the legs are the ani- 

 mal's jaws. Although aquatic species, they are related to the Scorpions, a division of 

 Spiders. See further, page 513. 



5. Crustaceans. 



The class of Crustaceans is divided into : — 



(1) Decapods (so-named from the Greek for ten-footed)^ as the Crabs, Lobsters, 

 Shrimps, usually having 5 pairs of feet. 



(2) Tetradecapods (named from the Greek for fourteen-footed), as the Sow-bugs and 

 Sand-fleas. 



(3) Entomostracans, irregular in number of feet, and usually without a regular series 

 of abdominal appendages. 



376-385. 



Articulates. — (1) Worms: 376, Arenicol.i marina, or Lob-worm (xj). (2) Crustaceans: 377, Crab, 

 species of Cancer; 378, an Isopod, species of Porcellio; 379, an Ampbipod, species of Orchestia; 380, 

 an Isopod, species of Scrolls (x \) ; 381, 382, Sapphirina Iris; 381, female; 382, male (x 6); 383, Trilobite, 

 Calymene Blumenbachli; 384, Cytbere Americana, of tbe Cypris family (xl2); 385, Anatlfa, of the 

 Cirriped tribe. 



In an early stage of development, many young Crustaceans have a 6-footed free- 

 swimming form, called a Nauplius^ 2 of the feet being functionally antennse and 4 of 

 them legs, the third pair afterward becoming jaws. All Entomostraca pass through this 

 Nauplius stage, and also a few of the higher kinds. 



Among the Decapods, Crabs are called Brachyurans, — from the Greek for short- 

 tailed, the abdomen being small and folded up under the body ; the Lobsters and Shrimps, 



