ON THE LOCOMOTOR SYSTEM OF ECHINODERMATA. 
35 
In Holothuria and Echinus the pedicels are provided with sucking discs, which are 
sometimes strengthened by a calcareous skeleton. In both, but especially in Echinus, 
they can be projected to a considerable distance beyond the surface. In Spatangus 
the feet are comparatively short, and although some have suckers, many are without 
them and end in simple rounded points, while others are either simply flattened at their 
apex, or flattened and split up into leaf-like segments. In Solaster and Uraster the feet 
are long, and terminate in large sucking discs, with the exception of a few at the end 
of each arm, which are pointed, and act as feelers. In Astropecten the feet are conical, 
devoid of suckers, and can only be projected about a quarter of an inch beyond the 
surface. In Ophiura the feet are even more pointed and shorter than those of 
Astropecten. Under the disc at the bases of the arms they are nearly as long as in 
Astropecten; but they gradually diminish in size from within outwards, until near the 
tips of the arms it is almost impossible to recognise them. 
§ III. Nervous System of Echinus. 
The internal nervous system of Echinus consists of five radial trunks, which may be 
traced from the ocular plates along the ambulacral areas external to the radial canals 
to the oral floor, where they bifurcate and unite with each other, so as to form a 
pentagonal nerve-ring. This ring lies between the oesophagus and the tips of the 
teeth which project from the lantern. Small branches leave the ring and supply the 
oesophagus, and lateral branches arise from the several trunks to escape with the 
pedicels through the apertures of the pore plates. Each trunk lies in a sinus 
(Plate SO, fig. 13, c) situated between the lining membrane of the shell (Plate 80, 
fig. 13, cl) and the ambulacral radial canal (Plate 80, fig. 13, e) ; the lateral branches 
which accompany the first series of pedicels through the oral floor are large and deeply 
pigmented; the other branches within the auricles are small; those external to the 
auricles gradually increase in size until the equator is reached, and from the equator 
to the ocular plates they again diminish. At the equator the trunk is wider than at 
either pole, and it is often partially divided for some distance at each side of the equator 
by a deep longitudinal fissure. When the nerve trunk, after being stained with chloride 
of gold or with osmic acid, is removed from its sinus, it is seen to be enveloped by a 
thin fibrous sheath. This sheath contains numerous large pigment cells, and has 
scattered over it irregular masses of protoplasm which have been deposited from the 
fluid of the neural sinus. 
When the sheath is removed the trank is seen to consist of delicate fibres and of 
fusiform cells (Plate 80, fig. 14) ; the cells consist of a nucleus and a thin layer of 
protoplasm, which projects at each end and terminates in a nerve-fibre. 
The lateral branches of the trunk escape along with, and are partly distributed to, 
the pedicels ; the remainder breaks up into delicate filaments which radiate from the 
base of the pedicel under the surface epithelium (Plate 80, fig. 13, l). When one of 
5 r 2 
