234 MORPHOLOGY OF INVERTEBRATE TYPES 



membranes of the third and fourth arm. In the adult male the 

 left fourth arm (the lower left arm often called in text-books 

 the fifth arm when the tentacle is counted as the fourth) is 

 hectocotylized; beginning with about the eighteenth to twentieth 

 suckers from the base of the arm the hectocotylization progresses 

 distally. It consists in an enlargement and swelling of the pedi- 

 cels and a reduction of the cups. In the thirtieth to thirty-fifth 

 suckers the cups disappear almost completely. The small 

 suckers at the end of the club gradually become normal again. 



On each side of the head is a large and well developed eye in 

 which one may easily recognize the transparent cornea, the 

 opaque iridescent iris, the pupil and the lens. In front of the 

 eye, near the edge of the cornea, is a small opening the aqui- 

 ferous pore which may be closed by a sphincter muscle. It 

 leads into a short aquiferous canal which opens by means of a 

 ciliated funnel into the outer chamber of the eye. On each side 

 of the head, beginning just behind the eye, is a fold of the skin 

 called the olfactory crest. Its free edge is covered up by the 

 mantle. On the lower surface of the squid, projecting forward 

 from under the mantle, is the funnel (infundibulum) often called 

 the siphon for the reason that its function is in part analogous 

 to that of the cloacal siphon of lamellibranchs. The two struc- 

 tures are, however, of different origin. The siphon of the lamel- 

 libranchs is part of the mantle, while the funnel of the squid is 

 part of the foot. The funnel is a hollow, somewhat flattened 

 conical tube open at both ends and attached to the head and 

 visceral mass. Its tip may be directed forward or backward at 

 will, and in consequence the current of water ejected through the 

 funnel may be forward or backward. The squid swims in the 

 direction opposite to that of the current. 



The rest of the body of the squid is completely hidden by the 

 mantle, which has the shape of a long cone with two fins. The 

 free ventral edge of the mantle, called the collar, is drawn out 

 into three more or less equidistant projections. The anterior or 

 upper projection marks the end of the pen, while the two postero- 



