202 



CHAPTERS ON EVOLUTION. 



ing gill-slits resembling those seen in the early life of all Verte- 

 brates. There seems little reason to doubt that this curious animal 

 is a survival of a once widely-represented type, which to-day exhibits 

 decline and decay, whilst preserving for us the important characters 

 of a common ancestor of several existing groups of animals. 



Ranking above the sea-stars, in respect of generally higher 

 organisation, we find a very numerous and varied assortment of 

 animals known as the Crustacea. The etymology of this latter term/ 

 might suffice to convey information respecting the typical repre- 

 sentatives of the group, inasmuch as the presence of a hard crust 

 or " shell " characterises the higher forms, as well as many lower 

 members of this class. Such higher forms are the crabs (Fig. 115), 

 lobsters, shrimps, prawns (Fig. 132), water-fleas (Fig. 116), and 

 their neighbours, which possess a "shell" although, as even a 

 tyro in zoology knows, the " shell " of the crab is a widely different 

 structure, in nature as it is in appearance, from the " shell " of the 

 oyster or whelk. The crab's shell is periodically slipped off its 

 body to admit of the animal's increase in size; whilst that of the 

 mollusc oyster, mussel, whelk, &c. is a permanent structure, 

 attached by muscles and other organic means to the animal's body, 

 growing steadily as bones grow in ourselves, and forming, therefore, 

 a much more important item of bodily belongings than does the 

 Crustacean's covering. But apart from the nature of the " shell," 

 the Crustaceans, as one may see in the jointed tail of the lobster 



or shrimp, are very 

 differently planned, 

 so to speak, from 

 the Molluscs. They 

 are " Articulate " or 

 " jointed " animals, 

 and naturally claim 

 insects, centipedes, 

 scorpions, spiders, et 

 hoc genus omne^ as 

 their relatives and 

 friends. Now, this 

 great Crustacean class 

 includes a very motley 

 and varied series of 



beings. At its head, as we have seen, are the lobsters, crabs, 

 shrimps, and prawns ; its middle-classes are represented by the 

 "water-fleas" (Fig. 116), whose name is legion; and its lower orders 

 are the barnacles (Fig. 117), the sea-acorns, the Sacculinas (Fig. 118), 

 and a host of allied creatures which certainly present us with the best 

 examples of degradation in the animal kingdom, in that they exist for 



FIG. 116. WATER-FLEAS. 



