SOFT-BODIED ANIMALS— THE MOLLUSKS 281 



There are quite a number of marine gastropods which can be recog- 

 nized by the spiral shell. Many persons have a very interesting hobby 

 of collecting these shells which are so abundant along the seashores all 

 over the world. Smaller shells may be used for necklaces and bracelets 

 as souvenirs for tourists, and the larger ones may be made into a variety 

 of table ornaments of great interest and beauty. 



Class — Cephalopoda 



This class name means "head foot" because the foot is wrapped 

 around the head, causing some confusion of the parts of each. 



The squid is a good example of the class. The squid has a some- 

 what torpedo-shaped body, with the visceral hump drawn up into a long 

 pointed shape and covered by the mantle, but not surrounded by a shell. 

 There is a vestigial remnant of the shell, the pen, embedded in the mantle 

 at the anterior edge, however, which serves to support the softer parts of 

 the body. The foot bears ten arms that are lined with suckers which 

 give the animal a good grip on anything around which it wraps these 

 arms. The squid has two eyes that have the iris, lens, and retina, such 

 as are found in vertebrate animals. This does not mean that the squids 

 are very close relatives of the vertebrates, but because of the many 

 other points of difference between the two, it is thought that the eye of 

 the squid has developed independently along the same lines as the verte- 

 brate eye. 



The squid sucks water up into its mantle cavity for respiration and 

 may use this same water for locomotion. There is a little tube leading 

 from the cavity called the siphon which can expel this water rather forci- 

 bly and propel the animal in the opposite direction using the principles 

 of jet propulsion. By pointing the siphon in different directions, the 

 squid can control the direction of movement, aided somewhat by a pair 

 of fins that project out to either side of the mantle. This method of loco- 

 motion causes them to move in a series of rapid darts when in a hurry, 

 but they can use the arms to crawl along in a more leisurely fashion. 



The squid not only has a pen, it also has ink, but these are not used 

 for writing letters. There is an ink sac which opens into the mantle 

 cavity and when in danger the squid will squirt a cloud of black ink out 

 into the water which acts in the nature of an underwater "smoke screen," 

 giving the squid an opportunity to escape. This is not always effective, 

 however, for the squid is very commonly found in the stomach of fish 

 and furnishes them with an important source of food. The squid is also 

 an important source of food for man. They are eaten in the Orient, in 



