STUDIES IN ANIMAL LIFE. 81 



of the body. The MoUusk, on the contrary, has no 

 skeleton, internal or external,* and its nervous sys- 

 tem does not form an axis. As a rule, both verte- 

 brates and articulates have limbs, although there 

 are exceptions in serpents, fishes, and worms. The 

 Mollusks have no limbs. Backboned, jointe/i, and 

 non-jointed, therefore, are the three leading charac- 

 teristics of the three types. 



Let us now glance at the fourth division — the 

 Eadiata, so called because of the disposition of the 

 organs round a centre, which is the mouth. Our 

 fresh-water vases afford us only one representative 

 of this type— the Hydra, or fresh-water Polype, 

 whose capture was recorded in the last chapter. Is 

 it not strange that while all the Eadiata are aquatic, 

 not a single terrestrial representative having been 

 discovered, only one should be found in fresh wa- 

 ter ? Think of the richness of the seas, with their 

 hosts of Polypes, Actiniae, Jellyfish, Starfishes, Sea- 

 urchins, Sea-pens (Pennatulce), Lily-stars {Comatu- 

 Ice), and Sea-cucumbers {Sblothurice), and then com- 

 pare the poverty of rivers, lakes, and ponds, re- 

 duced to their single representative, the Hydra. 

 The radiate structure may best be exhibited by the 

 diagram of the nervous system of the Starfishf on 

 page 79. 



* In the cuttlefish there is the commencement of an internal 

 skeleton in the cartilage-plates protecting the hrain. 



+ It is right to add that there are serious doubts entertained re- 

 specting the claim of a starfish to the possession of a nervous sys- 



D2 



