250 ^HE ANIMAL KINGDOM 



The lower jaw closes outside the upper jaw, resembling the reverse of a 

 parrot's beak. The nautilus also has a pair of large, protruding eyes 

 that form images. Each eye is a simple cavity (Fig. 14.16) with a small 

 hole opening to the exterior. Water is free to enter the cavity, which is 

 lined with a retina differentiated from the ectoderm. Images are formed 

 on the principle of the pinhole camera, a lensless system that requires 

 only a very small opening to a dark chamber with a light-sensitive back 



surface. 



The nautilus and its relatives, including the extinct ammonites,- 

 dominated the seas for many millions of years, dwindling to near ex 

 tinction at the end of the Mesozoic era. Throughout this time various 

 cephalopod groups showed tendencies to reduce the size of the shell and 

 make it an internal structure. Today most of the living members belong 

 to such groups. The squids (Fig. 14.14) are the most highly developed 

 in the direction of an active, swimming predator. The tentacles are 

 fewer in number, longer, and bear numerous suckers. The funnel is 

 closed into a complete tube, while the mantle, no longer confined 

 within the shell, has become a muscular pump that draws water in 

 around its free edge and expels it through the siphon. The squids can 

 match fish in speed and agility. The octopuses (Fig. 14.17) have gone 

 back to the ocean bottom where they crawl rapidly over the rocks and 

 swim only when chased. 



114. Loligo 



The common squids, Loligo pealei of the east coast and L. opalescens 

 of the west, grow to 8 to 12 inches in length and are frequently netted 

 in large numbers by fishermen and sold at market. A glance at their 

 streamlined shape and compact organs (Figs. 14.14 and 14.15) suggests 

 that the changes initiated by the nautiloids have been carried much 

 further in the squid. 



The squid is elongated like the gastropods, but in this case the 

 elongation remains straight. The body is covered by a thick muscular 

 mantle that tapers to a point. The shell is reduced to a pen, buried in 

 the upper portion of the mantle. The anterior part of the foot is com- 

 pletely disassociated from the rest and is intimately fused with the 

 head, forming a complete ring of eight tapered arms and two elongate 

 tentacles. The posterior part of the foot, much smaller than in the 

 nautilus, is fused into a tubular funnel attached to the lower side of the 

 head. The head is carried on a slender neck and fits snugly into the 

 opening of the mantle. In life it is locked in place by three articula- 



Figure 14.14. Lateral view of Loligo. 



