SOME AQUATIC WORMS, ETC. 171 



They are all rapid swimmers, and on that account are 

 rather difficult to study, but by following one for a 

 little while, it will usually settle down and begin to 

 seek food, and that is the observer's opportunity, un- 

 less he desires to kill the specimen, and study it after 

 death. 



The following Key leads to some of our common 

 forms : 



Key to Species of Ckcetonotus. 



1. Upper surface with neither spines, prickles, nor 



scales (a). 



2. Upper surface bearing scales only (b). 



3. Upper surface bearing spines or prickles only (c). 



4. Upper surface bearing both spines and scales (/). 



5. Upper surface bearing posterior spines and anterior 



prickles (g). 



a. Back smooth and naked, not furrowed, podura, 1. 

 a. Back transversely furrowed, sulcdtus, 2. 



a. Back covered with small hemispherical elevations, 



condnnus, 3. 



b. Caudal branches of moderate length, scales round- 



ed, loricdtus, 4. 



b. Caudal branches very long and jointed ; scales very 



small, rhombic, rhomboides, 5. 



c. Spines covering the entire upper surface (d). 



c. Spines not covering the entire upper surface (e). 



d. Spines long, mouth beaded, mdximus, 6. 



d. Spines short, mouth not beaded, Idrus, 7. 



e. Spines eight, in two longitudinal rows of three 



