A SQUID 125 
convenient for general use and will be employed hereafter in 
these directions. 
The mantle of the squid does not secrete an external shell as 
does that of the snail and the clam; in a long pocket on the 
upper side, however, is an elongate, horny structure, called the 
pen, which is secreted by the mantle and is the equivalent of 
the shell of other mollusks. 
Make a short shallow incision in the upper surface of the 
mantle, beginning with the collar. Turn the flaps aside and 
note the brown, horny pen lying beneath. Do not remove it at 
present, as the dissection of the parts beneath might be dis- 
turbed by its removal. 
Exercise 1. Make a drawing of the underside of the animal. 
Note that the arms may be divided into a right and a left 
group, each containing five arms. Observe a single arm; how 
many rows of suckers has it? Observe the structure of a 
sucker. Note the difference between the two long arms and 
the others in the place of origin and the arrangement of the 
suckers. 
The mantle cavity. Open the mantle cavity by a longitudinal 
incision through the thick mantle wall of the under side of the 
body to one side of the median line, running from the collar to 
the apex of the animal, taking care not to injure the delicate 
organs within. Notice, in the first place, that the collar is 
not attached to the head at any point of its circumference; 
and also that on the inner surface of the mantle, on the upper 
side of the body in the median line and also on each lateral 
surface, there is an elongate, cartilaginous structure which 
fits into a corresponding cartilage on the body, an arrange- 
ment which enables the collar to be applied very closely to 
the head. 
Place the animal in water with the head away from you and 
pin down the flaps of the mantle. Observe the soft visceral 
