APPENDIX. 375 



2. Sub kingdom of Articulates — so named with reference to the 

 fact that the body consists of a series of segments or joints, articulated 

 together ; and that all the legs, antennee, and other appendages are 

 likewise jointed (articulated). The body has one cavity containing both 

 the viscera and the principal nervous cord, the latter situated below the 

 alimentary canal. The species included are Insects, Spiders, Centipedes, 

 Crustaceans (or Crabs, Lobsters, Shrimps and the like), and Worms. 



3. Sub-kingdom of Mollusks — or, as the name implies, species having 

 soft fleshy bodies, which are characterized also by a simple bag-like struct- 

 ure, and by the absence of joints both from the body and all appendages. 

 As in Articulates, similar organs are repeated on the right and left sides 

 of a median plane, instead of around a central axis ; but there is no succes- 

 sion of segments in the body, or of corresponding ganglia (nervous masses) 

 in the nervous system ; and, consequently, Mollusks have not that compo- 

 site feature that characterizes and distinguishes Articulates. Examples are 

 the Oyster, Clam, Snail, Cuttle-fish, and Bryozoans (mentioned on page 

 105). Many of the species have shells, as an external covering; but 

 many also are without them. 



4. Sub-kingdom of Radiates, the subject of this note. 



5. Sub-kingdom of Protozoans, briefly described on a following page. 

 The division of Radiates is thus the lowest but one in the system 



of animal life, and its species are strikingly distinct from the higher kinds 

 in the radiate arrangement of the parts within and without. 



Radiates are of three Classes. 



1st. Polyps, whose characters have already been stated (p. 20 and be- 

 yond.) 



2d. Acalephs, or Jelly-fishes, or Medusae as many of them are called. 

 Acalephs are often nearly transparent and jelly-like in aspect, though not 

 in consistence. They have sometimes the shape of a disk, convex above, 

 or a hemisphere, or a bell-shaped spheroid, and vary in diameter from a 

 fraction of an inch to three yards or more. Attached either to the margin, 

 or to the under concave surface about the mouth, there are usually four 

 tentacles or groups of tentacular appendages, or a continuous fringe of 

 tentacles ; or there are other tasselings beneath the pellucid body ; and 

 these organs, like the tentacles and. some other parts of an Actinia, are fur- 

 nished with myriads of lasso-cells. The whole structure is as completely 

 radiate within and without as that of a Polyp ; but there are radiating, 

 and radiately branching, vessels passing outward from the stomach cavi- 

 ty instead of radiating compartments. Acalephs, or jelly-fishes, float in the 

 ocean, usually with the mouth downward, moving ordinarily by the con- 

 traction and expansion of the sides of the body. Hydroids (p. 101) are 

 sexless forms under one division of Acalephs ; they are usually attached, 

 and look like polyps. 



3d. Echinoderms. Examples of this class are, first, the star-fishes, or 



