268 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 hydrocoele 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 hydrocoele 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 arras 

 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. 



PI. XXII, figs, 151 — 154, show us that the aboral wall of the 

 perihsemal 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- 

 hsemal wall as in Asterids, we are brought face to face with a 

 most interesting effect which this produces on the nervous 

 system. Fig. 156 gives a section of the radial nerve-cord of an 

 Ophiurid. We notice two great masses of cells and fibres on 



