14 COMPARATIVE ANATOMY 



but the retina unlike that of vertebrates is not inverted. Altogether, the 

 cephalopods are much the most highly organized and active of the molluscs. 



The class divides into two orders. 



The tetrabranchiata, or four-gilled forms, have an external shell, coiled 

 in modern species like a snail's, but, unlike a snail's, divided into com- 

 partments, only the last and largest of which is occupied by the animal. 

 The chambered or pearly nautilus, of which there are only four species, 

 is the only survivor of a vast assemblage which flourished in the Paleozoic 

 and Mesozoic. Many of the earlier forms had straight shells. Alto- 

 gether, including the ammonites of the Mesozoic, some two thousand 

 fossil species are known. 



The dibranchiates have an internal shell or none. The group includes 

 the squids and cuttlefishes, octopus, and devil-fish. 



Phylum II. Arthropoda 



The similar metamerism of their bodies prompted Cuvier to group 

 annelids and arthropods together as articulata. 



Arthropods are metameric invertebrates with jointed legs, and covered 

 with a jointed chitinous exoskeleton usually beset with hairs. The 

 general organization of the body resembles, closely, that of annelids, 

 from which, it is believed, that arthropods have descended; but the 

 coelom is secondarily replaced by a pseudocoelom. The eyes are simple 

 and compound, both types often being present in the same individual. 

 The circulatory system is open, that is, the blood vessels open into the 

 lacunar spaces of the pseudocoelom. Comparative anatomy and embryol- 

 ogy suggest that, originally, each metamere possessed a pair of appendages, 

 which, however, in the course of phylogenesis have become greatly modi- 

 fied by specialization or by reduction. 



Species number 400,000 or four-fifths of all known animals. 



Owing to the differences in the method of respiration, arthropods are 

 divided into (i) Branchiata, which, having adapted themselves to aquatic 

 Hfe, breathe by means of gills; these gills, usually, being attached to their 

 appendages; and (2) into air breathing Tracheata, which use ectodermal 

 air-tubes that ramify throughout the tissues. 



Class Crustacea 



The crustaceans include all the water-breathing arthropods. Besides 

 the crabs, lobsters, crayfish, shrimps, and prawns, together with several 

 thousand species of smaller creatures allied to them, the group contains 

 the familiar sowbugs of damp woods, five hundred species of barnacles, 

 and some five thousand species of minute creatures, water fleas and the 

 Hke, in size from i cm. down to the Hmit of comfortable visibility, about 

 half of which are parasitic. 



