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TEXT-BOOK OF ZOOLOGY 



later on.) Hence, the star-fish (like all echinoderms and coelenterates) 

 is constructed on a radiate or radially-symmetrical type, in which upper 

 and lower surfaces are different, but bilateral symmetry is very slightly 

 indicated. 



D. Body Covering. 

 In the soft integument are deposited numerous calcareous plates, 

 which afford an important and necessary protection to this slow and 

 defenceless creature. In spite of this armour, however, both the body 

 and arms are very movable, owing to the calcareous plates moving easily 

 upon each other. After death, however, they render the body rigid. 

 (Compare with arthropods, p. 308.) 



jr.Ai//. 



Section through the Disc and One of the Arms of a Star-Fish (Diagrammatic). 



K., Calcareous integumentary plates (shown black), elevated into numerous spines (S.) and 

 several pincers (pedicellarise) (Z.) ; M., madreporic plate ; Stk., stone canal ; Kk., circular 

 canal ; Lk., longitudinal canal. On the left the latter, which extends along the bottom of 

 the left arm, is shown in transverse section as a white circular spot. From it the lateral 

 canals (not visible on the right) pass off to the ampullae (B.) and the pedicels (F.). 

 M., mouth shown widely opened ; Ma., stomach ; A.Ma., a branch or csecal pouch of the 

 , stomach extending into the right arm ; on the left of the stone canal the stomachal pouch 

 is shown in transverse section. 



On the outside these plates are produced into numerous spines, which 

 likewise protect the animal from attack. Among these structures are 

 also found numerous minute pincer-like bodies (pedicellariae), borne upon 

 movable stalks, the function of which is not yet definitely ascertained. 

 The majority of naturalists are inclined to regard them as organs for 

 cleansing the skin, for both star-fishes and sea-urchins are always 

 remarkably clean and white, in spite of the fact that their spiny coat 

 is well adapted for harbouring all kinds of plants and minute animals — 

 indeed, far more so than the shell of crustaceans (see p. 422), which we 

 frequently find almost overgrown with foreign bodies. 



