64 Part second. 



the Sea-urchins these plates are fixed together so closely, that the spheri- 

 cal body seems to consist of a solid inextensible substance; in the Star- 

 fishes 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 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 long- 

 thin arms, and some with branching arms swing them about like whips, 

 and so progress. The skin of the Sea-cucumbers is enlireh^ leathery^ 

 and contains instead of the calcareous plates innumerable extremely 

 Small pieces of carbonate of lime , often of very curious shapes such 

 as anchors and rosettes ; they are consequently able to straighten their 

 bod}' 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 in life they are covered. But on dead specimens 

 — the attendant has always some at hand — one can easily see the 

 immense number of these plates : in large examples a hundred thousand 

 may be present ! — How can Sea-urchins and Starfishes move about in 

 the water? In answer to this question we must ask the reader to ob- 

 serve carefully some specimen, which is attached to the glass front of 

 the tank. He Avill then notice on the side towards the glass numbers 

 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 ; when thej^ have fixed themselves by their sucker they con- 

 tract again and draw the animal toward the point of attachment. The 

 water necessary for the expansion of the feet is taken into the body by 

 a special system of canals (which have nothing to do with the blood- 

 system) leading to every one of the feet. These feet are equally im- 

 portant to th(; Sea-urchins to catch its prey. If any animal upon which 

 they can feed comes into their vicinity, they protrude some of these feet 

 till they reach its body. Should it not notice in time the attack which 

 is being made upon it, it is lost; other sucker-feet rapidly follow, and 

 soon the prey is woxind round with hundreds of fine tlireads, and is 

 slowly drawn to the mouth of the Sea-urchin. All struggle is in vain, 

 for if one or two sucker-feet are torn aw^ay others take their place, and 

 soon the prey gives up the fight and is devoured. But, so as not to 

 be recognized too soon in thus stalking their prey, many Sea-urchins 

 cover themselves on their upper surface with all sorts of stones, shells, 

 and pieces of sea-weed , and are thus more often successful. — The 

 mouth of the Sea-urchins and Starfishes is on the lower surface of the 

 body, and is therefore only visible Avhen they lie on their back, or are 

 attached to the glass on the front of the tank. Many Sea-urchins have 

 five strong teeth of very complex structure; others live always buried in 

 tiie sand and swallow it, so as to digest whatever organic substances it 



