30 
Tank No. 1. 
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 to 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 only is entirely leathery, and contains instead of the calcareous 
plates small pieces of carbonate of lime, often of very curious shapes 
such as anchors, stars, 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 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 will 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 they have fixed themselves by their sucker they contract again 
and draw the animal toward the point of attachment. The water neces¬ 
sary 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 important 
to the Sea-urchin 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 folloAV rapidly, and soon the 
prey is wound round with hundreds of fine threads, 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 away others take their place, and soon the 
prey gives up the fight and is devoured. But, so as not to be recog¬ 
nized too soon in thus stalking their prey, many Sea-urchins cover them¬ 
selves 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 when they lie on their back, or are attached to the glass on 
the front of the tank. Many Sea-urchins have strong teeth of very com¬ 
plex structure with which they can crush even hard objects to pieces. 
Others live always buried in the sand and swallow it, so as to digest 
whatever organic substances it contains, and then throw it out again. 
The Starfishes have no teeth, but the walls of their alimentary canal secrete 
such strong juices that they can kill animals which they have caught 
