130 AMERICAN JOURNAL 



2. Strongly developed senses to discover the danger, and 

 swiftness of movement in order to avoid the attack of enemies ; 

 nocturnal or spelunchal habits. 



3. Hard covering of the skin, as Dasypus, Testudo, Ostracion, 

 shells of Mollusca and Echinodermata, or spines, as Erinaceus, 

 Hystrix, Diodon, Murex, Echinus, &c. 



4. Forming nests or cells for the protection of the offspring ; 

 digging holes, or forming tubes, as the larva of Phryganea, or 

 the shell of the female Argonauta. The difference between con- 

 structions formed by the mouth or by the skin seems not to be 

 very considerable. 



5. By imitation of the colors of the ground where they live : 

 moulting polar animals, most Insects ; or by shape resembling 

 lifeless bodies, as twigs (Phasma3, larva of Geometrge). 



6. Great fertility, by which the species is protected from de- 

 struction, but is a proof that the individual is not provided with 

 skill to provide for its offspring : Entozoa, Plants. 



The want of hard protective skin or calcareous plates may in 

 general be considered a character of superiority. The greatest 

 number of species with hard covering is found among the lowest 

 classes of the Vertebrata : Reptilia and Fishes. Among Mol- 

 lusca all the Diat )cardia are provided with a shell or tube. 

 Among Exophillia a few (pelagic !) Heteropoda and Onchidiop- 

 sis only are naked. 



The Androgyna offer the greatest number of naked forms ; 

 as Aplysiffi, Gymnobranchia, Pellibranchia, and the only naked 

 terrestrial Mollusca (Liraax, Veronicella, &c.) Among Cepha- 

 lopoda, the Tetrabranchiata, which are provided with the most 

 developed shells, are considered the lowest. It seems, therefore, 

 that in general, species which need no hard protecting covering 

 are of a superior stamp, if they are not degenerative forms pro- 

 tected by the means indicated under 6. 



The naked Mollusca appear always to be the largest in size 

 in their respective groups. 



XVI. The degree of mutual dependence of individuals affords 

 good characters for indicating relative rank. 



1. Individuals free, never attached or radiated (holozoic 

 Dana). 



a. Living in pairs, taking charge of their young. 



