386 E. W. MACBRIDE. 
concentration of this in the neighbourhood of a greatly deve- 
loped sensory tentacle. The support of this tentacle by the 
arm is a secondary matter, as we have already learned—a fact 
which comes out still more clearly in Crinoid development. 
There the primary hydrocele lobes develop into long free 
tentacles covered with sensory hairs. At a very late period 
(later than any which Seeliger observed) these primary tentacles, 
according to Perrier (17) become applied to five outgrowths 
of the body-wall; these latter immediately bifurcate to form 
the ten arms, and so the free tips of the tentacles are situated 
each in the angle between a pair of arms. Seeliger (18) adduces 
this last fact to show that the primary tentacles are not the 
same as the primary hydroceele lobes of Asterids, forgetting 
that the point where a pair of arms diverge corresponds to the 
tip of the Asterid arm, since in Antedon there are ten arms 
which have arisen by dichotomy from five. 
The epithelium of the water-vascular system in fig. 150 
shows an interesting feature; the cells have developed muscular 
tails which are arranged longitudinally, and the important point 
is that these myo-epithelial cells persist as such for a 
considerable period of free life. 
Pl. 29, figs. 151—154, show us that the aboral wall of the 
perihemal space also gives rise to muscles. These connect 
one ambulacral ossicle with its fellow of the opposite side, and 
serve, by approximating these to one another, to close the ambu- 
lacral groove. Figs, 151 and 152 show us that here again we 
have, in the first instance, to do with myo-epithelial cells. 
Muscles connecting one ossicle with its successor and prede- 
cessor are also present, but very much more feebly developed. 
In Ophiurids, however, as is well known, they are most power- 
ful, and this point gives the key to nearly all the peculiarities 
of this group as compared with Asterids. Presuming, as we 
fairly may, that these muscles are developed from the peri- 
hzemal wall as in Asterids, we are brought face to face with a 
most interesting effect which this produces on the nervous 
system. Tig. 156 gives a section of the radial nerve-cord of an 
Ophiurid. We notice two great masses of cells and fibres on 
