OM THE ECHINODERMATA. 
267 
development of which leads to a strange series of metamorphoses, 
until the final or parental figure is attained. The order is divided into 
several families, each containing many genera. The common Star¬ 
fish (Uraster ruhens), the Sun Star (Solaster), the Bordered Star 
(Astropectenj, and the Cushion Star ( Goniaster), are representatives of 
the order. 
Echinoidea. —The sea urchins are enclosed in a calcareous box of 
marvellous structure. The body is spheroidal, oval, or depressed, 
without arms, the whole body being shut up in its calcareous skeleton. 
They have a distinct mouth, situated at the under surface, and 
sometimes armed with a most complicated set of jaws and teeth; in 
other forms the mouth is edentulous, and the intestine opens in various 
parts of the body, often opposite the mouth, sometimes in a groove on 
the upper surface, or on the posterior border, or underneath the 
margin. The shell consists of twenty columns of calcareous plates. 
Ten of these, small and narrow, are called ambulacral, and ten, large 
and broad, inter-ambulacral; and between these two systems of plates 
ten other narrow columns of small plates are placed, between the 
pieces of which ten rows of holes are formed for the passage of vertical 
tubular sucking feet. The surface of the plates supports a number of 
tubercles, with a round polished surface, on which are placed spines of 
various sizes, shapes, and dimensions, in the different families of this 
group. The articulature between the tubercle and spine is that of a 
ball and socket joint, and in some a small ligament passes from one to 
the other, just as we observe in the thigh bone of mammals. Besides 
the large species, there are many secondary spines, and the surface of 
the plates is moreover covered with a more or less abundant develop¬ 
ment of fine granules. At the summit of the test is the apical disc, 
composed of five ovarial plates, perforated for the passage of the 
ovarial and seminal tubes* and five ocular plates for lodging the five 
eyes. One of the ovarial plates, the right anterolateral, carries the 
madreporiform tubercle, which is in connection with a sand canal. 
Scattered over the surface of the test are a great number of curious 
forms called Pe dicellar ice, which we shall show in the microscope 
demonstration. These remarkable organs play an important part in 
the life-history of the urchin and starfish. These bodies are 
supported on a long stem, which is attached either to the skin or shell; 
the movable head consists of two or three prongs, which move upon 
each other, and form a forceps, the blades of which open and close 
upon any body which irritates the tegumentary membrane. They 
appear to serve the animal for removing foreign materials from 
the surface of the shell, by picking it away from the spines and 
integument. They are incessantly in motion, and will grasp a pin 
placed near the open forceps which the head forms. The mouth of the 
common Echinus is provided with a complicated armature of jaws and 
teeth, forming what is called Aristotle’s lantern, which consists of 
five long three-sided triangular sockets with jaws united together, with 
their apices pointing downward, so as to form a pyramid. Each jaw 
