76 ASTERIAD.E. 



merit of the spines covering the surface and bordering the 

 avenues. These characters represent respectively the di- 

 gestive, the motor, and the tegumentary systems. As 

 might be expected, those derived from the last, it being 

 characteristic of the order, furnish the most constant cha- 

 racter. Useful characters may be drawn from the form 

 and arrangements of the spinous eyelids ; but the madre- 

 poriform tubercle, which several Continental authors have 

 suggested as a good source of distinctive character, fur- 

 nishes none of importance, either generic or specific. The 

 sources of specific character are various in the different 

 genera. The spines of the skin furnish the most impor- 

 tant ; colour, probably, may be ranked next ; and pro- 

 portions of parts last, though in some genera of great im- 

 portance. 



The causes which seem to affect the distribution of 

 Mollusca on the British coasts do not appear to influence 

 that of the true Starfishes. There are as many kinds 

 found in Cornwall as in Zetland belonging to the same 

 genera, and even identical as species. A very feAv seem 

 to prefer the western to the eastern shores of Britain, or 

 vice versa. The multiplication of individuals is of course 

 affected by the nature of the sea-bottom. When we look 

 abroad, climate appears to affect but slightly the forms of 

 this tribe. The Cushion-stars, perhaps, present some in- 

 dications of tropical influence, and of a concentration of 

 their genus in the eastern seas ; but among other Star- 

 fishes neither form nor colour are remarkably modified by 

 geographical position. Of the numbers and distribution 

 of true Starfishes in the primaeval seas we cannot well 

 judge, their substance and structure being such as to be 

 ill adapted for preservation in a fossil state. 



The British species of Asteriadse may be arranged under 



