CRUSTACEANS. 153 



B. Respiration aqueous or by means of gills, — unless the species is * 

 so minute that the surface of the body is equivalent to a gill in- its 

 action. Essentially water-species, living either in water or moist 

 places. * 



2. Crustaceans. — The body in two parts, — an anterior, called the 

 cephalothorax, consisting of a head and thorax, the posterior called 

 the abdomen ; locomotion by means of jointed organs. Examples: the 

 Crab, Lobster, Shrimp. 



3. Worms. — Worm-like in form, without any division into cephalo- 

 thorax and abdomen ; the body fleshy ; no jointed legs, though 

 often furnished with tubercles, lamellae, or bristles. Examples: the 

 Earth-worm, Leech, Serpula, Intestinal Worm. 



The water-species of Articulates commence in the Silurian, and 

 are here further explained. 



Crustaceans. — Among Crustaceans there are three orders : — 



The^rs^, or highest, ten-footed species, or Decapods; as Crabs (fig. 

 171) and Lobsters. 



The second, fourteen-footed species, or Tetradecapods (figs. 172, 173, 

 174). 



The third and lowest, irregular in number of feet, and unlike 

 the Tetradecapods, also, in not having a series of appendages to 

 the abdomen : the species are called Entomostracans t from the Greek 

 for insects with shells. 



(a.) Among the Decapods, Crabs are called Brachyurans, — from the Greek for 

 short-tailed, the abdomen being small and folded up under the body ; the Lob- 

 sters and Shrimps, Macrourans, — from the Greek for long-tailed, the abdomen 

 being as long as the rest of the body. 



(b.) Ampng the Tetradecapods, figs. 172, 174 represent species of the tribe of 

 Isopods (a word meaning equal-footed), and fig. 173 of that of Ampthipods (feet 

 of two kinds, abdominal as well as thoracic). Fig. 172 is the Sow-bug, com- 

 mon under stones and dead logs in moist soil. Fig. 174 is the Sand-flea, abun- 

 dant among the sea-weed thrown up on a coast. In figs. 172, 174 (Isopods), 

 the abdomen is abruptly narrower than the cephalothorax; its appendages 

 underneath are gills. In fig. 173 (Amphipod) the abdomen is the part of the 

 body after the eighth segment ; its appendages are swimming legs and stylets, — 

 the gills in the Amphipods being attached to the bases of the true legs, and not 

 to the abdomen. 



(c.) Among Entomostracam the forms are very various. The absence of a 

 series of abdominal appendages is the most persistent characteristic. The eyes 

 in a few species have a prominent cornea ; but in the most of them the cornea 

 is internal and there is no projection. In the Cyclops group the species have 

 often a shrimp-like form, as in fig. 175, though usually minute. Sometimes the 

 male and female differ much in form : 176 is male and 175 female of the Sapphi- 

 rina Iris ; a b is the cephalothorax, and b d the abdomen. There are legs on 



