Chap. III.] GROUPS OF HlGHER METAZOA. 57 



coiled on itself that its orifice comes to lie at the 

 side, and in the anterior half of the body. 



The difficulties arising from our imperfect know- 

 ledge, and the generalised characters of the lower 

 forms which are associated together under the head of 

 the Vermes, disappear, for the most part, when we 

 rise above them in the scale of animal organisation. 



No one, for example, can fail to see that a starfish 

 is no close ally of a crayfish, or a snail of a frog ; on 

 the other hand, a sea-urchin, and a starfish are as 

 clearly allied to one another as is the crayfish to the 

 crab, the mussel and snail to the octopus, and the 

 shark to the frog, the pigeon, or the rabbit. 



While the bases or origins of these several forms 

 are obscure enough, the apex stands sharply out, and 

 we may compare the four series of forms of which 

 mention has just been made to four great branches 

 arising from a common trunk. Each of these branches 

 may be called a phylum. In one the body wall 

 becomes richly impregnated with calcareous salts, 

 which sometimes form projecting spines, the original 

 bilateral symmetry yields to an acquired radial one, 

 and locomotion is typically effected by a special series 

 of suckers connected with a system of water-tubes ; 

 this is the phylum of the Echiisodermata or star- 

 fishes. In another the soft body becomes invested in 

 and protected by a hard shell which is secreted by a 

 special outgrowth of the body called the mantle ; 

 the ventral surface is drawn out into a muscular foot, 

 and a series of delicate filamentous processes grow out 

 on either side of the body ; this is the phylum of the 

 Mollusca, or shell-fish. 



In yet another series we find a closer resemblance 

 to the Annulata than is exhibited in any other of the 

 higher phyla. Some or all of the metameres become 

 provided with appendages, which are most often 

 jointed, and one or more of these pairs of appendages 



