40 ZOOLOGY. 
“This apparatus communicates with another tube which penetrates from. 
~ the dorsal surface downwards, having its opening shut by a perforated plate 
called the madreporic body, which in starfishes is always seen in the angle 
between two of the rays; so that we have here an hydraulic apparatus of 
a very complicated nature.” (Agassiz.) Through this series of vessels the 
water flows in both directions, either downwards through the upper aper- 
ture, or upwards through the tubular feet; subserving in its course the 
functions of locomotion and respiration. The water which fills the general 
cavity is admitted through the numerous minute perforations of the exterior. 
‘“The heart is placed along the calcareous tube which arises from the 
madreporic body, and the blood-vessels form circular rings around the 
entrance of the stomach, from which and to which the radiating arteries 
and veins move.” (Agassiz’s Lectures, and his Letter to Humboldt in 1847.) 
There are also movable spines upon the lower surface which assist in 
locomotion. | 
When food is taken, the animal bends its rays towards the mouth, so as 
to form a cup-shaped cavity, when the food is gradually moved to the 
mouth. There is no vent distinct from the mouth. The rays, when lost 
by accident, can be reproduced, and it is asserted that if a ray with part of 
the mouth be detached, it will form a new animal. The stomach is central 
and sends off two branching divisions or ceca in each ray. There is an 
English law which imposes a fine upon fishermen who do not kill a species 
of Asterias which is said to destroy oysters. 
Agassiz has discovered that starfish, after their eggs are laid, take them 
up and retain them below the mouth between their suckers ; and when they 
are forcibly removed to some distance, the animal will approach and take 
them up again, showing a remarkable instinct in so low an animal. — 
We pass from Asterias (pl. 76, fig. 60), through the pentagonal form 
Astrogonium (fig. 61) to Agassiz’ genus Culcita, which resembles the last 
somewhat in shape, except that the five sides are convex instead of concave, 
so that the outline is more nearly circular, approximating the circular and 
oval forms of the next family. 
Orpver 3. Ecainmpea. This order includes the oval or circular. bodies 
known as sea-eggs, sea-urchins (owrsen in French), the skeleton of which is 
a calcareous crust composed of twenty equal or unequal rows of polygonal 
plates pierced by various pores. The mouth is beneath, and armed or 
unarmed, central or sub-terminal ; the vent is distinct, and varies in position, 
being formed beneath and above, and when beneath, marginal or towards 
the centre. The generative pores are four or five, placed around the summit. 
When the animal dies, the integument (including the spines with which 
it is covered, and which present much variety in form and size) is soon 
lost, leaving the calcareous shell which protects the interior soft parts. The 
integument of the calcareous portion not only secretes the shell, but extends 
in a thin layer over the solid spines, which are thus formed layer by layer 
as the animal increases in size. 
The circular form of these animals might at first induce an observer to 
doubt their affinity with the more star-shaped Asteridea, but if the rays of 
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