CHAPTEE X 



STARFISHES, SEA UECHINS, ETC. 



STILL passing up the scale of animal life, we now come to the 

 Echinodermata the other sub-kingdom which we have already 

 referred to as forming, with the Coelenterates, the old division of 

 Eadiata. The term Echinoderm signifies ' hedgehog skin,' and is 

 applied to the group on account of the fact that the majority of its 

 species possess a skin that is either distinctly spiny, or exhibits 

 numerous more or less defined prominences. This skin is also 

 supported and hardened by the deposit of little plates or spicules 

 of carbonate of lime, all joined together so as to form a kind of 

 scaffolding or ' test ' for the protection of the animal ; and this 

 secretion of carbonate of lime is not always confined to the outer 

 skin, for, in some cases, it occurs in the walls of the internal organs 

 as well. 



Most of the animals of this sub-kingdom display a regular 

 radiate symmetry ; that is, the parts of their bodies are arranged 

 regularly round a common axis, and the arrangement is usually 

 a five-fold one, as may be observed in the case of the common 

 Five-fingered Starfish of our coasts (see Plate IV.)i and it is worthy 

 of note that this radiate disposition of parts is not merely external, 

 but that, as in the case of anemones and jelly-fishes, it also obtains 

 within, and determines the arrangement of the internal organs. 

 Further, although this radiate symmetry characterises the adult 

 animals of the group we are considering, yet some show a tendency 

 towards bilateral symmetry (parts arranged equally on two opposite 

 sides of a common axis), while this is the rule, rather than the 

 exception, with the early stages or larvce of these creatures. 

 Observe, for instance, the larva of the common Brittle Starfish, 

 the adult of which species exhibits an almost perfect radiate 

 symmetry, and we see something more than a mere trace of a two- 

 sided disposition. 



