SECT. vi. ECHINODERMATA. 169 



SECTION VI. 



ECHINODEEMATA. 



THIS class consists of five orders, all of which are 

 marine. They are, with one exception, creeping ani- 

 mals, and the whole class is remarkable for having 

 most of their members and general structure either in 

 fives or multiples of five. Their skin is hardened by 

 calcareous deposits, sometimes of beautiful microscopic 

 structure : they have a digestive cavity, a vascular fluid 

 system, and some distinct respiratory organs, so that 

 they are comparatively of a high grade. 



Echinodermata Astero'idea. 



The Astero'idea, or Star-Fishes, which are the highest 

 order, form two natural families, the Stelleridae and 

 Ophiuridse, which comprise twenty-two genera. 



The simplest form of the Stelleridse is the common 

 star-fish, with its flat regularly five-sided disk. A tough 

 membrane, strengthened by reticulated calcareous mat- 

 ter, covers the back, and bends down along the sides, 

 while the under- side of the body or disk, on which the 

 animal creeps, is soft and leathery, with the mouth in 

 its centre. In the other genera, although the body is 

 still a flat, five, equal-si^ed disk, the angles are extended 

 into long arms, broad whence they diverge from the disk, 

 but decreasing rapidly in width to their extremities, so 

 that the animal is exactly like a star with five long, 

 equal, and flexible rays. 



