12 THE SHELL-FISH OF THE COAST. 
A form of squid very similar to the one just de- 
scribed, and largely replacing it in the northern 
OMMASTREPHES SAGITTATUS, ~ 
waters, is the Ommastrephes 
sagittatus, in which the ten- 
tacular arms are of compara- 
tively short length, and the 
cornea of the eye perforated, 
so as to permit of the entry 
of sea-water to the lens. 
While rare with us, this 
animal sometimes appears 
in the northern waters in 
immense shoals, following 
in the wake of the mackerel, 
- which constitutes its selected 
food. In this condition it 
does not hesitate to enter the 
pounds and weirs, or to nav- 
igate between the piles of 
wharves, darting with the 
swiftness of an arrow into 
the midst of its prey, and 
pouncing upon the neck of 
a selected victim. In this 
pursuit of the mackerel the 
squid may be observed to 
change color frequently, adapting itself in tone, 
by an intuitive manipulation of the pigment-bodies 
(chromatophores), to the surroundings which it 
traverses. The squid, like many other. cuttle- 
fishes, is in a measure nocturnal in its habits, and 
is thought to be fond of gazing at the moon. 
