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 l. 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 



