Ciiap.I. APPEARANCE OF CILIARY CHORDS. 13 



(PI. II. Figs. 19, 23, 25). Since the formation of the mouth, and the change of 

 position of the first -formed opening to an eccentric one, we find the mouth and 

 anus placed on one side of the larva. These openings present, at this stage (PI. 

 II. Fig. 17), the same relations as the mouth and anus of Clypeaster and Scutella-like 

 Echinoids, while at a much earlier period they are more like Pygorhynchus. 



If we now return to the water-system, we find that the two- diverticula (iv,w'), 

 mentioned above (PI. II. Figs. 15, 16), have entirely separated from the digestive 

 cavity (PL II. Fig. 18), and are now distinct cavities, having no connection whatever, 

 either with the cavity from which they originated, or with one another; one of 

 these cavities is entirely closed (w), the other (iv) connects with the surrounding 

 medium, by means of a very small opening, the dorsal pore (b, PI. II. Fig. 23, and 

 isolated Fig. 17). Such is the appearance of an embryo at the close of the second 

 day after fecundation. 



Midler never knew the origin of the water-tubes ; in his last paper only, 

 he becomes aware that they are independent at first, but subsequently unite. 

 It must be remembered, in reading his earlier papers, that he sets at rest, in his 

 last memoir, the doubts he expressed concerning the independence of the two 

 branches of the water- tubes; in fact, to obtain a clear conception of Midler's views, 

 it is advisable to read his last memoirs first, to be able to adopt at once the 

 corrections he himself makes, during the laborious course of his investigations. The 

 problematic bodies, however, still remained a puzzle to him, even at the time of 

 his last memoirs, as he was never aware that they were simple diverticula of the 

 digestive cavity, and were finally transformed into the two independent branches of 

 the water-tubes, uniting, in subsequent stages of growth, to form the Y-shaped water- 

 system. Van Beneden saw, in the young Bipinnaria (Brachina Van Ben.), that the 

 water-tubes are at first separate, but he did not trace their mode of formation, and 

 no other observer has since returned to this subject. 



Appearance of the Chords of vibraiile Cilia. — The cilia, spreading over the whole 

 surface, which moved the embryo so rapidly at first, have almost entirely disappeared, 

 and are no longer capable of propelling such a large mass ; consequently, at this 

 last-mentioned stage (PI. II. Fig. 20), the larva is very sluggish, advancing but 

 little, and rotating slowly about a longitudinal axis at the same time. During the 

 third day, the movements become still more sluggish ; it is then that we find 

 the first appearance of the organs which are to propel the larva in future. The 

 general outline does not change during the third day; the principal transformations 

 are the greater bending and extending pf the oesophagus and alimentary canal, 

 the increase in size of the mouth, of the water-tubes, and the appearance of slight 

 projections, small clusters of vibratile cilia, near the anterior and posterior sides of 

 the mouth, which are the beginning of rows, extending in older larvae in continuous 



