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cilia next become restricted to a single band round the ovum, 
and at the same time the depression extends inwards, forming 
a complete alimentary canal, with mouth, pharynx, stomach, 
short intestine, and anus. The plane of the ciliated band does 
not coincide with the direction of the alimentary canal, and 
both are bent down anteriorly. The larva is thus, at first, quite 
bilaterally symmetrical, i.e. it is divisible into a right and a left 
half, which are equal and corresponding one to the other. In the 
adult, as we have seen, the symmetry is radial, not bilateral. 
The next modification is the outgrowth of the ciliated band into 
long processes, in which condition the larva is called a jpluteus , 
and is furnished with a delicate and complex skeleton of anasto- 
mosing calcareous rods. As yet, however, we have no trace of 
the future echinus. This complex organism, so well furnished 
with a digestive apparatus, so actively locomotive, and sup- 
ported by so beautiful a skeleton, is not destined for permanent 
existence, nor can it reproduce its kind. It is but, as it were, 
the nurse of the secondary and true larva, to which it is com- 
pelled to yield an important part of its own organisation, and 
the perfecting of which is its own inevitable destruction. 
Soon on one side of the stomach of this unlucky primitive 
larva there arises a mass of formative substance, or bud, and 
at the same time a depression appears at a point of the external 
surface, which deepens, becomes ciliated internally, and forms 
a circular canal at its lower end, and this sends out five 
other canals around and amongst the bud of formative sub- 
stance just mentioned. These five canals are the future five 
longitudinal, ambulacral vessels, and the developed “ bud ” 
becomes the young echinus. The rest of the structure of the 
“ pluteus,” or primitive larva, withers, shrivels, and dies, its 
stomach having been enclosed and cut off from the gullet by 
the secondary larva (or young echinus), which develops its 
own mouth, intestine, and anus, together with spines, suckers, 
pedicellariae, &c. 
The further growth of the echinus, until it attains its extreme 
size, is also effected by means quite different from those em- 
ployed by the typical animals already described. 
The lobster casts its dense coating, the nautilus forms new 
chambers, the mussel fresh internal layers of shell substance, 
but in the echinus each separate plate is capable of indi- 
vidual enlargement by means of the perisoma which invests 
it on all sides; while absolutely new plates are developed 
as required towards the oral and apical poles of the enlarging 
corona. 
The echinus may serve as the type of a large and important 
group of animals, consisting, besides the sea-urchin and sea- 
eggs, of star-fishes, brittle-stars, sea-cucumbers, and crinoids or 
