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 

 betw^een two of the rays ; so that we have here an hydraulic apparatus of 

 a very complicated nature." (Agassiz.) Through this series of vessels the 

 M^ater flows in both directions, either downwards through the upper aper 

 ture, or upwards through the tubular feet ; subserving in ite course the 

 functions of locomotion and respiration. The water which fills the general 

 cavity is admitted through the numerou's minute perforations of the exterior. 

 "Tlie 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 caeca 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 {pi. 76, jig. 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. 



Okder 3. EcniNiDEA. This order includes the oval or circular bodies 

 known as sea-eggs, sea-urchins {oursin 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 summ.it. 



When the animal dies, the integument (including the spines wnth 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. 



Tlie 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 

 244 



