108 INVERTEBRATE ANIMALS. 
of little muscular bladders () placed at their bases, and they 
can thus be thrust far out beyond the shell, into which they can 
be again withdrawn at the will of the animal. However long 
the spines may be, the animal can protrude the tube-feet to a 
still greater length; and by the combined action of the little 
suckers at their extremities locomotion is effected with mode- 
rate rapidity, considering the bulk of the body. 
The digestive system in the Echinus consists of a mouth 
armed with a curious apparatus of calcareous teeth, which 
opens into a gullet, which in turn conducts to a distinct 
stomach. From the stomach there proceeds a long and con- 
voluted intestine, which is attached to the interior of the 
shell by a delicate membrane or “ mesentery,” and terminates 
in a distinct vent. The surface of the mesentery, as well as 
that of the lining membrane of the shell, is richly ciliated, and 
thus serves to distribute the fluids of the body-cavity to all 
parts of the body. In this way, also, respiration is subserved, 
though it is probable that the chief agent in this function is 
to be found in certain specialised portions of the ambulacral 
system. The proper blood-vascular or circulatory system con- 
sists in its central portion of two rings (fig. 43, 2 d, f) placed 
round the opposite ends of the alimentary canal, and united by 
an intermediate muscular cavity or heart (ec). The nervous 
system consists of a gangliated cord placed round the gullet, 
and sending five radiating branches along the ambulacral areas. 
The sexes are distinct, but in both the reproductive organs are 
in the form of five membranous sacs placed in a radiating 
manner in the interambulacral areas, and opening at the 
genital plates. The embryo of the Achznus is at first a little 
free-swimming ciliated organism, and it passes through an ex- 
traordinary development, which can only be alluded to here. 
In its later stages it was originally described as a distinct 
‘animal under the name of “ Plufeus” (fig. 43, 1). In this state 
the larva is a curious easel-shaped body, with a distinct ali- 
mentary canal and an internal calcareous skeleton, and exhibit- 
ing distinct bilateral symmetry. The remarkable point, how- 
ever, about its further development is, that the young Echznus 
is developed out of only a fortzon of the Pluteus, and the 
greater part of the latter, including the skeleton, is cast away 
as useless, 
The majority ot the sea-urchins are found at moderate depths 
