Scientific Lectures. 227 



Thus in the clams and mussels, as common examples, we have a 

 tileshy expansion called the " foot," and this is simply an organ of loco- 

 motion. The head is not developed, only a mouth, which has no 

 power of seizing food, and this part has no power of projecting ])eyond 

 the edges of the bivalve shell that protects it. In tlie snails we have 

 .a more specialized creeping disk or foot, and now the mouth can be 

 •protruded from the shell, and the creeping disk is not only used for 

 locomotion, but for prehension, as in the beach cockle (natica) for 

 example. This has the power of seizing their prey and holding it, 

 while the mouth, now armed with special hard parts for biting and 

 rasping, can readily reduce its food. In the highest members of the 

 mollusca ( cuttle fishes, squids) the same creeping disk is highly speci- 

 alized into long arms furnished with suckers, and now we find the 

 head possessing a pair of highly organized eyes, and the mouth armed 

 with two parrot-like beaks, and also the so-called tongue for rasping 

 its food. Thus we have in the lowest class simply locomotion, in 

 the next class locomotion and prehension, and in the highest class 

 locomotion, prehension and aggression, for the cuttle fish is an aggres- 

 sive animal, and with its long arms can seize fishes of large size. 



If we now turn to the articulates we shall find the principle of 

 cephalization well illustrated in the relative rank of the classes. Thus 

 in the lowest class, the worms, the body is made up of a series of rings 

 identical in shape and strn(?ture. In the next class, the Crustacea, 

 rthe number of segments or rings composing the body is less, and now 

 a specialization of junction is manifested by some of the rings toward 

 the head ; for example, there is one group of crustaceans in which the 

 anembers of the group have seven pair of legs and four pair of mouth 

 ■organs or jaws. The group next above, comprising the highest Crus- 

 tacea, such as the lobster and crab, have only five pair of legs and seven 

 pair of mouth organs, thus while the body has lost two pair of legs 

 the head has gained two pair of legs, simply by the conversion of the 

 locomotive organs to a higher and more special use. 



If we run up in the articulates we shall find that the locomotive 

 appendages are reduced in number, in the spiders only four pair of 

 legs are seen, and in the insects, the highest class in the articulates, 

 only three pair of legs are present, and now the body is so highly 

 specialized that the head is highly specialized, also; the thorax is 

 separated from the hinder division of the body, and called the abdo- 

 jnen. 



If we now look at the vertebrates, we shall find the locomotive 



