Echinoderms. 63 



nophora and Siphonophora, are nearly all voracious robbers ; often swal- 

 lowing animals , which one would suppose migfyt easily overcome them. 

 Inside the cavity of Beroe or^in the pendant stalk of a Meduse are often 

 seen small fish which the apparently delicate orgenism dissolves and 

 digests. 



In the bay of Naples the following Ctenopora occur: the barrel- 

 shaped, comparatively tough Beroe (Fig. 85) ; the much more delicate 

 Callianira (Tig. 87); Eucharis ( Fig. 86 ) consisting chiefly of water: 

 and lastly the curious Venus' d Girdle, CestUS Veneris (Fig. 88). All of 

 these attract the attention of the observer by the beads of light which 

 seem to run over the body in various directions , displaying all the co- 

 lours of the rainbow. This curious play of colours is caused by innume- 

 rable little plates, which are placed in vertical rows one above the other, 

 and are moved up and down with great rapidity. By them the rays of 

 light are reflected in such a way that the colours of te rainbow, which 

 make up white light, appear separately. These plates, arranged in rows 

 like the teeth of a comb, 'have caused these organisms to be called Cte- 

 nophora ( comb-bearers ). 



ECHINODERMS. 



This group of animals comprises the Sea-urchins , Sea-stars . Sea-' 

 cucumbers , and Feather-stars. They are especially interesting to those 

 not acquainted with marine life, for no member of the group occurs either 

 in fresh water or 'on land. But the Echinoderms differ from most ter- 

 restrial and freshwater animals in a further way. If the lay mind be 

 rather doubtful as to details of anatomical structure , it knows that, for 

 example, birds, fishes and insects possess a head; that they are provided 

 with wings, fins or legs. They have a mouth, and eyes, and many other 

 organs with which we are all familiar from every day observation: but 

 how should he recognize these in a sea-urchin or in a starfish? These 

 animals live, therefore they must eat; but where is their mouth ? where 

 are the limbs with which they grasp their prey ? They live in the sea, 

 but how do they move about? Do they crawl? Can they see and hear? 



First of all it must be remembered that .Corals and Medusae, for in- 

 stance, can do very well without head and tail , arms and legs , fins or 

 wings, and so can the Echinoderms. Still , the latter have a much more 

 complex structure than the former. Even their outward form is most 

 variable. Some are almost spherical (Fig. 7), others are flat like a leaf 

 (Palmipes, p. 5), others again star-shaped (Fig. 1 and 3); some are cylin- 

 drical, something like a cucumber in shape (Fig. 10) ; and lastly some 

 have the form of a flower attached by a short stalk, and seem to possess 

 sepals and petals (Fig. 5). The skin of these animals contains a large 

 number of different but regularly arranged calcareous plates. In the Sea- 

 urchins these plates are fixed together so closely, that the sperical body 

 seems to consist of a solid in extensible substance ; in the Starfishes the 

 plates are more loosely connected one to the other , and allow the body 

 a certain amount of flexibility so that the arms (the term applied to the 

 rays of the starfish) can be bent udwards and downwards and from side 



