58 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



become specially modified to the purpose of the 

 mouth. This phylum, which we will call that of the 

 Arthropoda, might, if constancy of nomenclature 

 were not a matter of convenience, be more appro- 

 priately designated as the Cnathopoda (Lankes- 

 ter). Lastly, there is an important phylum for 

 which, in the light of recent researches, it seems well 

 to adopt some other name than the ordinary desig- 

 nation of Vertebrata. This phylum is remarkable 

 for the development along the dorsal area of a rod, 

 which, at first hollow, subsequently becomes solid, 

 and forms a primitive and, in some cases, permanent 

 support for the overlying nervous system. In recog- 

 nition of the presence of this cord we will speak of 

 the phylum as that of the Chordata ; here are in- 

 cluded the degenerated Tunicates, the primitive and 

 somewhat modified Lancelet (Amphioxus), and the 

 great group of fishes, reptiles, birds, and mammals in 

 which a vertebral column, more or less well de- 

 veloped, encloses and protects the spinal cord ; these 

 are the true Vertebrata (Balfour). 



It is a matter of little importance which of these 

 phyla is first considered in greater detail, but, as the 

 most aberrant are the Echinodermata, it is, perhaps, 

 convenient to dispose of them first of all. 



One of the best known types of the Echino. 

 dermata is presented to us by the starfish (Asterias), 

 in which no bilateral symmetry is at first apparent in 

 the adult, though it is quite well marked in the larva. 

 There is a central rounded disc from which are 

 given off five rays or " arms ; " in other words, we 

 have the bilateral symmetry overshadowed by an 

 acquired radial symmetry (Fig. 21). On the prin- 

 ciples on which we have already worked, this mode 

 of symmetry in a freely moving animal is not, at once, 



