164 



MAlfUAL OF THE MJLLUSCA, 



quires great force to remove them. At other tim.es they dart 

 tail first, with the rapidity of an arrow, from one side of the 

 pool to the other, at the same instant discolouring the water 

 with a dark chesnut-brown ink. They also escape detection 

 by varying their tints, according to the nature of the ground 



over which they pass. In the 

 dark they are slightly phospho- 

 rescent." (Darwin.)* Professor 

 E. Forbes has observed that 

 the octopus, when resting, coils 

 its ventral arms over its back, 

 and seems to shadow forth the 

 argonaut's shell. 



In the male octopus, the third 

 right arm is more developed than 

 the corresponding arm on the 

 left side, and terminates in an 

 oval-shaped plate (Fig. 38, c), 

 marked with numerous trans- 

 verse ridges, between which are 

 pits. A muscular fold of skin 

 passes from this plate down the 

 dorsal margin of the arm to 

 the web at its base ; the mar- 

 gin is rolled up, and forms a 

 covered passage through which 

 the spermatophore is probably 

 transmitted to the terminal plate. 

 The arm is permanently at- 

 tached, and is developed in a 

 free state from a cyst, A. 



Distribution : universally 

 found on the coasts of the tem- 

 perate and tropical zones; 46 

 species are known ; when 

 adult they vary in length from 

 1 inch to more than 2 feet, according to the species. 



Fig. 38. Octopus carena $ , Ver. 



A, Side view, showing cyst in place of 

 third arm. 



B, Ventral side of an individual more 

 developed, with the Hectocotylus C. 



Suh-genus. Tremodopus (Ohiaje), PL I., Pig. 3. 



Name from two large aquiferous pores [tremata) on the back 

 of the head. 



* "Journal of a Voyage round the World." The most fascinating volume of 

 travels published since Defoe's fiction. 



