ARTICULATES. 



RADIATES. 



BRIEF REVIEW OF THE SYSTEM OF LIFE. 419 



The Articulates having jointed limbs, including the terrestrial species and Crustaceans, 

 are now generally removed from the Worms, and placed in a separate grand division, 

 called Arthropods (from the Greek &pdpov, joint, and irovs,foot). But the typical Worms 

 and the Arthropods are alike in consisting of a series of segments, each normally having 

 its nervous ganglion ; and in this fundamental feature, which is more important than their 

 differences, both sections are far removed from MoUusks and Brachiopods, which are non- 

 articulates, the body and its appendages having no joints. On this account the old division 

 of Articulates still has importance. The relations of Insects are even closer, sti-ucturally 

 and embryologically, to Worms, than to Crustaceans, notwithstanding their jointed limbs. 

 This relation of Insects to Worms is shown by the resemblance of the larves to Worms ; 

 while Crustaceans, by the same evidence, are proved to be most nearly related to the 

 precursors of Worms. 



The grander divisions of Invertebrates are as follows : — 



1. Arthropods. 



a. The terrestrial or Tracheate species : 

 1. Insects ; 2. Myriapods ; 3. Arachtiids. 



h. The aquatic or Branchiate species : 

 4. Limuloicls ; 5. Crustaceans. 



2. Worms. 



NON-ARTICULATE S. 3. Mollusks ; 4. Molluscoids (including Brachiopods and 



Bryozoans). The non- segmented Worms might here make 

 another subdivision. 



5. ECHINODERMS. 



). CcELENTERATES, including Hydrozoans (or Medusae and 

 Hydroids), and Actinozoans (or Polyps). 



7. Spongiozoans, or the animals of the Sponges. 



8. Protozoans, Amoe.hoids, Bhizopods, Badiolarians, Monads, 



and other Flagellates, etc. 



1. Arthropods. 



The Tracheates have spiracles (breathing-holes), a vascular system for inside air- 

 circulation, and one pair of antennse, or none ; they include Insects, Myriapods, Arachnids. 



The Branchiates have gills for the aeration of the circulating fluid, or perform this 

 function through the general surfaces of the body or its foliaceous appendages. The spe- 

 cies are Crustaceans, Limuloids, and Pycnogonids. 



1. Insects. 



Having the body in three parts, that is, a distinct head, thorax, and abdomen ; and 

 only three pairs of legs : as Hymenopters (Ants, Bees, Wasps) ; Lepidopters (Butterflies, 

 Moths) ; Coleopters (Beetles) ; Dipters (Flies) ; Neuropters (Dragon-flies, May-flies) ; 

 Orthopters (Grasshoppers, Locusts, Cockroaches) ; Hemipters (Cicada, Squash-bug, 

 Aphis); Thysanura (Podura, Lepisma) . 



2. Myriapods. 



Having a worm-like form, regularly articulate body, and numerous pairs of legs ; 

 part have the body flattened, and one pair of legs to a segment or somite, the Chilopoda, 

 which include the Scolopendra and other Centipeds ; and others have the body nearly 

 cylindrical, and two pairs of legs to a segment, the Diploopoda, which include the lulids 

 and other Millepeds. 



