MOLLUSKS 87 



twelve to thirty years ; and the average length of life of 

 the snail appears to be from two to five years. Certain it 

 is that mollusks have numerous enemies besides man which 

 prevent multitudes from living lives of normal length. 

 Birds, fishes, frogs, .and starfishes beset them continually, 

 and many fall a prey to the ravages of internal parasites or 

 to other mollusks. Under ordinary circumstances the shell 

 is sufficient protection, and the spines disposed on the sur- 

 face in many species render the occupant still less liable to 

 attack. Many snails carry on the foot a horny or calcare- 

 ous plate known as the operculum, which closes the en- 

 trance of the shell like a door against intruders. Certain 

 noxious secretions poured out from the skin also serve as a 

 means of defense, and many Nudibranchs (Fig. 50) bear 

 nettle- cells on the processes of the body, which probably 

 render them distasteful to many animals. Finally, there 

 are numerous clams, mussels, snails, and slugs whose colors 

 harmonize so closely with their surroundings that they al- 

 most completely baffle detection, and enable them to lead 

 as successful a life as those provided with special organs of 

 defense. 



90. Cephalopods. The animals belonging to this class, 

 such as the squids and cuttlefishes (Fig. 52), are by far 

 the most highly developed mollusks. They are of great 

 strength, capable of very rapid movements, and several spe- 

 cies are many times the largest invertebrates. In almost 

 every case there is a well-defined head bearing remarkably 

 perfect eyes, and also a circle of powerful arms provided 

 with numerous suckers which aid in the capture of food 

 (Fig. 52). Posteriorly the body is developed into a pointed 

 or rounded visceral mass which to a certain extent is free 

 from the head, giving rise to a well-marked neck. Some 

 forms, such as the squids (Fig. 52, upper figure), are pro- 

 vided with fins which drive the animal forward, but in com- 

 mon with other cephalopods they are capable of a very rapid 

 backward motion. By muscular movements water is taken 



