SUB-KINGDOM V. ECHINODERMA. 



STAX-FISH, ETC. 



BY F. A. BATHER, M.A., F.G.S., ETC. 



THIS is one of the main groups of the animal kingdom, and the animals 

 contained in it, such as the star-fish (Figs. 1, 2), brittle-stars (Figs. 5, 6), 

 sea-urchins (Figs. 7, 8), feather-stars (Figs. 13, 15), and sea-cucumbers (Figs. 

 9-12), are well marked off from all other animals by certain well-defined 

 characters. The first is the feature that has suggested the name of 'the 

 group, namely, 

 the deposition in 

 the lower layers 

 of the integu- 

 ment of a crystal- 

 line deposit of 

 carbonate of 

 lime. This may 

 be in the form of 

 minute spicules 

 (Fig. 11, s), of 

 separate plates, 

 or of larger 

 plates and 

 ossicles closely 

 joined together 

 into a more or 

 less rigid test 

 (Figs. 7 and 13). 

 In all cases it is 

 characterised by 

 its trellis - work 

 structure, and by 

 the ease with 

 which it may be 



split in certain definite directions. Such deposition of carbonate of lime 

 may also take place among the internal organs of the animal. 



The second feature is the radiate structure so obvious in the ordinary 

 star-fish (Fig. 1). The number of rays is generally five, and the external 

 and internal organs of the animal are variously affected in the various classes 

 of echinoderms by this five-rayed symmetry. The Echinoderma resemble the 

 Mollusca, the Arthropoda, the Vertebrata, and many other groups, in 

 possessing a separate gut, shut off from the rest 'of the body-cavity, as well 



653 



Fig, 1. A STAR-FISH (Asterias rubens) FROM UPPER SURFACE. 

 Two thirds natural size. 



