114 STARFISH GALLERY. 



each other like vertebrae, the series extending from the mouth to the 

 end of the arms. The groove in which the ambulacral feet are 

 arranged is uncovered. 



Typical specimens of this Order are exhibited in Cases 2 & 3, in 

 which the great variety of form in the genus Asterias (283, 286-292) 

 and beautiful examples of Acanfhaster (295) are shown. Cases 6 & 7 

 contain specimens illustrating the curious habit of self-mutilation 

 possessed by so many Echinoderms. Specimens (317-324) illustrate 

 the power possessed by some Starfishes of throwing off their arms, and 

 forming from each separate arm a new individual. The end of the 

 arm nearer the disk is capable of giving rise, by budding, to a fresh 

 central disk, and to four or more arms. From the appearance 

 presented by such Starfishes in an early stage of the process, the 

 term Comet-form is usually applied to them. 



It is not known whether the setting free of an arm requires an 

 external stimulus, but it is obvious that this process of reproduction 

 is of great advantage to the species ; it may, moreover, be noted 

 that it is seen at its best in forms that have no protecting plates, or 

 defensive spines ; it has not, in other words, been observed in 

 Pentaceros (361-370) or in Astropecten (390-401). Cases 9-11 

 contain a fine series of Pentaceros. 



Ophiueoidea, or " Brittle-stars." — These Echinoderms appear to 

 resemble the ordinary Starfish * ; but they differ in having the organs 

 of digestion, respiration, and reproduction confined to the disk, the 

 arms having merely the function of locomotor organs. The arms 

 therefore are more slender and cylindrical in form, and are sharply 

 distinct from the disk ; the separate joints consist of two central 

 ossicles, which leave only a narrow canal between them, and these are 

 covered above, below, and at the sides by specially developed investing 

 plates ; the lateral plates bear spines, which are always comparatively 

 short and delicate, as compared with the spines found at the sides of 

 the arm in starfishes. 



The ambulacral ossicles appear to be formed on three types. The 

 simplest condition is that in which one surface of an ossicle has two 

 pairs of slight convexities, and the opposing surface of the neigh- 

 bouring ossicle has two pairs of slight concavities. This allows of 

 a moderate amount of movement of the ossicles, while uniting the 



* The Asteroidea and Ophiuroidea may be united under the name Stelli- 

 formia. 



