THE HEAD. 



The head of the squid consists of the primitive or true head and a portion 

 of the foot. The portion representing the true head is slightly longer than broad 

 and in transverse section is reniform. It may be compared to the stem of a 

 lance whose blade is represented by the arms. The head is divided into three 

 portions viz.: 1) a wedge-shaped dorsal portion consisting of the skull and the 

 ganglia protected by it (oesophageal and optic ganglia) ; 2) a similar but more 

 acutely wedge-shaped ventral portion consisting of an oval pharynx or "buccal 

 bulb" enclosed by the muscular sheath formed by the interwoven muscle fibres 

 at the base of the arms; 3) an hourglass-shaped central portion formed by the 

 two hemispherical eyes whose convex inner surfaces almost meet at the center 

 of the head and so separate the skull and ganglia behind from the pharynx and 

 arms below. The majority of the muscle fibres which attach the arms and 

 pharyngeal sheath to the skull arise from the cephalic and pedal processes of the 

 skull and pass to the arms through the triangular spaces above and below the 

 eyes. The pharyngeal sheath formed largely by these fibres is conical or tent- 

 shaped and is attached to the lateral ligaments as well as to the ventral pro- 

 cesses of the skull. The oesophagus passes back from the pharynx through a 

 venous canal which also holds a pair of arteries and the salivary duct, and which 

 is a dorsal continuation of the cavity between the sheath and the pharynx. 

 Each eye with its ganglion is enclosed by an unsymmetrical , helmet-shaped 

 capsule formed of skin and muscle and attached to the margin of the optic cup. 

 Beginning at the cephalic process , the line of attachment of the capsule to the 

 skull can be traced outward along the upper edge of the skull , then downward 

 along its outer edge , inward along its lower edge as far as the pedal process , 

 and finally upward along the ventral edge of the lateral ligament. The outer 

 wall of the capsule forms the cornea and almost the entire lateral surface of the 

 head while the inner wall separates the eye from the sac for the peduncle of 

 the tentacle. At the junction of the outer and the inner walls of the capsule , 

 the membrane which connects the third and fifth arms is attached to the capsule 

 so that to a superficial observer the arms seem to be attached to the eye. The 

 cavity of this capsule is the outer chamber of the eye and is in communication 

 with the exterior by means of the "aquiferous pore", a tubular ciliated canal 

 situated at the ventral edge of the cornea. 



