375 
TIIE ECHINUS, OR SEA-URCHIN. 
dissolved, and the body drawn out at the poles. These are 
“ sea-cucumbers.” These have a vermiform body, a flexible in- 
tegument with calcareous particles, an anus which is terminal 
in position, and an ambulacral region greatly in excess of the 
antambulacral region. The primitive larva, however, is, in this 
group, not in the form of a pluteus, but vermiform. Moreover, 
sometimes there are but three rows of ambulacra instead of five, 
the lantern is represented only by rudimentary alveoli and rotulse, 
there are no spines and no pedicellariae, and the generative 
organs are unsymmetrical, consisting of a bundle of blindly- 
ending tubes, opening on one side of the neck by a solitary 
aperture. This order contains the only monoecious form of the 
class — namely, the genus Synapta. 
The third order (fig. 7) is named Asteridea, and contains the 
star-fishes. Here the body is mostly stellate, though sometimes 
it is discoidal. The so-called arms are really parts of the 
body, into which extend sacculated processes of the alimentary 
canal. The ambulacra lie in deep grooves on the under surface 
of the so-called arms, and only on their under surface ; and thus 
the ambulacral region is only coextensive with the antambulacral 
region, instead of being in excess of it. The integument is 
strengthened by thick and strong calcareous opicles. The 
primitive larva is vermiform, and the so-called arms are some- 
times more than five in number. In many there are pedi- 
cellariae, but these have only two jaws. There is no dental 
apparatus in the mouth. The madreporic tubercle is inter- 
radial and conspicuous, and its canal, sometimes hardened and 
jointed, is termed the “ sand canal.” The nervous system is 
essentially like that of the echinus, and the diverging nerves 
are placed superficially to the ambulacral canals, and terminate 
each in an eye surrounded by movable spines. 
The fourth existing order consists of the Ophiuridea , or 
“ sand stars ” (fig. 8). These animals have real arms, distinct 
from the body (which is termed the calix ), sometimes branching 
and sometimes provided with lateral processes. The ambulacral 
region is coextensive with the antambulacral region. The in- 
tegument is calcareous, but not provided with pedicellariae. 
There is no anus. In one important point these sand stars 
resemble the echini, namely, in having the primary larva in the 
form of a pluteus. The ambulacral vessels run along just be- 
neath the ventral surface of the arms, but here (unlike the 
star-fishes) they are sheltered within the skeleton, not merely 
placed in grooves. 
The last and most aberrant order (fig. 9) of existing eehino- 
derms consists of the Grinoidea , all of which pass at least the early 
stages of their existence rooted on a stalk. Such forms abounded 
in that vast period during which primary and secondary rocks 
