PART II. 



SYNOPSIS OF THE GENEEA. 



Chapter I. 



CLASS I.-CEPHILOPODA. 



The cephalopoda are rejiresented by tlio common squid, tlio 

 nautilus, and the ammonite ; forms Vsiilo, which, most of us are 

 more or less familiar. They possess a more complicated struc- 

 ture than any other group of the mollusca ; but in this respect 

 they are much inferior to the Yertebrate animals, in whom 

 the setting apart of particular organs for the jDerformance of 

 distinct functions is developed to so high a degree. "We cannot 

 trace a series of gradatijnal forms between the highest cepha- 

 lopod and the lowest vertebrate ; but we can descend from the 

 more to the less specialised forms of mollusca, which ultimately 

 merge in one direction in such creatures as Fasciola, among 

 entozoa ; and in another direction, to forms like VorticeUa, 

 through the intermediate genera — Fcdicellina, among the 

 Bryozoa, and Peroplwra among the Ascidians. It is conse- 

 quently much easier to define the higher than the lower 

 boundaries of a great primary group. The points of analogy 

 between the cephalopods and the vertebrates are the internal 

 skeleton, the similarity in the form of the blood corj)uscles, and 

 in the capillary structure of the portion of the circulatory 

 system situated between the arteries and veins. 



The cephalopods move partly by means of a series of long 

 muscular arms arranged round the mouth, partly by means of 

 fins, or flaps, attached on each side of the body, and partly l)y 

 the forcible exj)ulsion of water through a tube or siphon. 



Unlike most of the mollusca, they are symmetrical animals, 

 having their right and left sides equally develoi)ed. Their shell 

 is usually straight, or coiled in a vertical plane. The nautilus 

 and argonaut alone (of the living tribes) have external shells , 

 the rest arc teiTaed " naked cci-)halopods," because the shell is 

 internal. They have powerful jaws, acting vertically, like the 



