Starfishes and Urchins. 6i 



VI.— STARFISHES AND URCHINS. 



These animals are known scientifically as Echinoderms, a 

 name which means " spiny sl\in," from tlie fact tliat tlie skin 

 contains calcareous matter deposited in the form of plates or 

 of spicules (a term wliich has been defined above, page 28), 

 often with spines. They are also called radiate animals, because 

 the5' present a radial instead of a bilateral symmetry, i.e., 

 their body is not divisible into two equal halves, but is made 

 up of several ra\'s ; tlie bndy shows no trace of the segmen- 

 tation which is so characteristic a feature of the Crustaceans 

 and the Annelid wiirms. 



The Common Starfish, Astcnas rubcns (froiitispiece) ait'ords a 

 good example of an Echinoderm. Tlie body is enclosed in a tough 

 but flexible skin, supported by plates, and consists of a central 

 disc, from which five finger-shaped arms radiate. The plates 

 of the skin are particularly well developed on the lower side of 

 the arms, where they are (iMong and arranged in two rows, 

 meeting at an angle so as to gi^'e rise to deep grooves, bordered 

 by spines. The niduth is (in the \'entral side, and the ^'ent 

 nearly in the middle on the dorsal side of the disc. I^ocomotion 

 is effected by means of peculiar tubules ending in sucker-like 

 discs, which, arranged in a double row in each groove of the 

 arm, function as feet. These little feet are liolkAv, and their 

 interior is in communication with a number of canals conveying 

 sea-water, which enters the system through minute holes pierced 

 in a single plate situated on the upper surface of the disc between 

 the bases of two arms. The position of this plate, which occurs 

 in the majority of Echinoderms, tends to show that the radial 

 symmetry of these animals is more apparent than real. When 

 the starfish moves, the tubular feet are protruded and adhere 

 to any surface by means of tlie sucker-lilvc discs with which 

 tliey are furnished. The arms are hollow, and contain pro- 

 longations of the gut and other organs. .Islerias rithens has 

 an average span of eight inches ; the; colour varies much, 

 some specimens being brownish red, others orange, some even 

 violet. The much smaller starfisli, Asterina gihhosa (frontispiece) 

 is often met with, and may be easily recognised by the lesser 



