SOME AQUATIC WORMS, ETC. 185 



3. A similar animal, but with the dorsal and lateral 

 surfaces closely covered with small hemispherical 

 elevations arranged in oblique lines, and giving 

 the creature a peculiarly neat and attractive ap- 

 pearance. Icthydium concinnus. 



3. Ch.'Et6notus (Figs. 134, 135.) 



"Bristle-back," the literal meaning of the word, 

 seems to be a misnomer when those forms are con- 

 cerned in which the bristles are replaced by scales, 

 but the structure is such that the only resting place 

 for these creatures is in the genus Chsetonotus. The 

 following key leads to a few of the common species. 

 A. Upper surface bearing scales only; scales 



rounded, loricatus, i. 

 A. Upper surface bearing spines or prickles only (a). 

 A. Upper surface bearing both spines and scales (^). 

 A. Upper surface bearing posterior spines and an- 

 terior pricTiles (c). 

 a. Spines long, covering the entire upper surface; 



mouth beaded, mdximus, 2. 

 a. Spines short, covering the entire upper surface; 

 mouth not beaded, Idrus, 3. 



a. Spines not covering the entire upper surface [d). 



b. Back with a subcentral, transverse hedge of large 



spines; scales double, acanthodes, 4. 



b. Back without distinct spinous hedge; scales not 



•double, spinifer^ 5. 



c. Spines in four transverse rows, five spines in each, 



acantKdphorus, 6. 

 c. Spines in transverse rows, less than five spines in 

 each, endrmis, 7. 



