164 MANUAL OF THE MOLLUSCA. 
quires great force to remove them. At other times 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 shghtly phospho- 
rescent.”’ (Darwin.)* Professor 
Ei. Forbes has observed that 
the octopus, when resting, coils 
its ventral arms oyer 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 tc 
the web at its base; the mar- 
gin is rolled up, and forms a 
covered passage through which 
the spermatophore is probably 
transmitted tothe 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- 
Fig. 38. Octopus carena &, Ver. 
pee eae cw. shows. evel. in place of nerate and tropical zones; 4€ 
B, Ventral side of an individual more species are known; when 
d j : a) oe : 
eveloped, with the Hectocotylus C Daa they vary in length from 
1 inch to more than 2 feet, according to the species. 
Sub-genus. Tremoctopus (Chiaje), Pl. I., Fig. 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. 
