284 



COMPARATIVE ZOOLOGY. 



maxillae, which works like the "screw" of a propeller. 

 The heart is a single oval cavity, and drives arterial blood 

 — a dusky fluid full of corpuscles. The alimentary canal 

 consists of a short gullet, a gizzard -like stomach, and a 

 straight intestine. 



Crustaceans pass through a series of strange metamor- 

 phoses before reaching their adult form. They also peri- 

 odically cast the shell, or moult, every part of the integu- 

 ment being renewed ; and another remarkable endowment 

 is the spontaneous rejection of limbs and their complete 

 restoration. Many species are 

 found in fresh water, but the class 

 is essentially marine and carnivo- 

 rous. 



Of the numerous orders of this 

 great class we will mention only 

 four : 



1. Cirripeds, distinguished by 

 being fixed, by having a shelly 

 covering, and by their feathery 

 arms (cirri). Such are Barnacles 

 (Zepas) and Acorn-shells (B alarms), 

 so common on rocks and timbers 

 by the sea-shore. 



2. Entomostracans, which agree 

 in having a horny shell and no abdominal limbs; repre- 

 sented by the little Water-fleas (Cyclops) of our ponds, and 



Fig. 251.— Idotea robusta: a Te- 

 tradecapod. U. S. coast. 



Fig. 252.— Amphithoe maculata: a Sand-flea. 



the Brine-shrimps (Artemia), and many others. The King- 

 crabs (Limulus) and the extinct Trilobites were formerly 



