'J 2 Part second. 



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 hfe, for no member of the 

 group occurs either in fresh water or on land and the visitor meets 

 them as quite new forms. 



Some are almost spherical (fig. 7), others are flat hke a leaf 

 {Palmipes, p. 4), others again star-shaped (figs, i and 3); some 

 are cylindrical, something like a cucumber in shape (figs. 9, 11); 

 and lastly some have the form of a flower attached by a short 

 stalk, and seem to possess sepals and petals (fig. 6). 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 spherical body seems to consist 

 of a solid inextensible substance; in the Starfishes the plates 

 are more losely 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 upwards and downwards 

 and from side to side. (These movements are best seen, when a 

 starfish is lying on its back and tries to turn over on its under 

 surface. At the request of the visitors the attendant will turn 

 a starfish on its back.) The Brittle-stars are able to perform 

 snake-like movements with their arms, and can move along with 

 great agility. The Feather-stars can even swim by using their 

 long thin feathered arms like whips. The skin of the Sea-cucum- 

 bers is entirely leathery, and contains instead of the calcareous 

 plates inumerable extremely small pieces of carbonate of lime, 

 often of very curious shapes such as anchors and rosettes; they 

 are consequently able to straighten their body and bend it again 

 in any direction. 



In the Sea-urchins and Starfishes one can scarcely observe 

 these calcareous plates on account of the large number of spines 

 and other processes with which they are covered in life. But on 

 dead specimens — the attendant has always some at hand — 

 one can easily see the immense number and definite forms of 

 these plates which compose the round dwelling. 



How can Sea-urchins and Starfishes move about in the water? 

 In answer to this question we must ask the reader to observe 

 carefully some specimen which is attached to the glass front of 

 the tank. He will then notice on the side towards the glass num- 

 bers of fine threads which bear at their extremities each a small 

 sucker. These threads are very mobile, stretch themselves and 

 contract again like so many worms, and are easily fixed to the 

 glass. These "sucker-feet" are hollow, and the animal forcing 

 water into them causes them to elongate considerably. They are 

 then fixed in the right direction to some firm object and by shorten- 



