18 EMBRYOLOGY OF THE STARFISH. Part I. 



III. Fig. 6, ww') ; and the water which enters into the right tube through the 

 dorsal pore (PL III. Figs. 2, 5, 7, b) can pass into the other branch on the opposite 

 side of the stomach, through the fork at the oral extremity, and not round the 

 stomach, where the water-tubes simply touch, but do not communicate. The small 

 tube leading from the dorsal pore to the main branch of the water-system widens 

 and becomes funnel-shaped as it approaches the main tube. The dorsal pore is 

 cut obliquely across the end of this small tube, giving it an elliptical shape. By the 

 time the two branches of the water-system have joined (PI. III. Fig. 6) at the oral 

 extremity of the larva, it has assumed an entirely different outline from any we 

 have met with in the former species. The anal extremity is very much flattened, 

 the corners of the anal plastron project slightly beyond the general outline, the 

 indentations have become very distinct, the oral plastron has grown rectangular with 

 rounded angles and concave sides, the oral triangular opening leads into a deep 

 pouch. The sides of the body are marked by three strong indentations (PL III. 

 Fig. 8). The oral extremity of the water-system changes rapidly from a rounded 

 to a pointed outline (PL III. Fig. 8, ww) ; it advances more and more towards 

 the oral extremity. In proportion as the dorsal region projects beyond the oral 

 plastron, the water-system extends into this projection, sending off, at the same time, 

 two branches leading into small appendages (PL III. Figs. 10, 11, /, /), (only de- 

 veloped in more advanced larvse), which have, in the adult larvse, a peculiar 

 structure (PL IV. Figs. 4, 5, 6). 



Changes of Form of the Larva. — The prominent changes now going on are only 

 changes of degree. The larva has completely lost its cylindrical shape, and even 

 the pear-shaped form it assumed afterwards ; it has become rectangular, with deep 

 indentations, gradually assuming the character of short arms. The transformation 

 from the pear-shaped (PL III. Fig. 1) to the rectangular flattened larva, with 

 undulating outline (PL III. Fig. 6), can readily be traced by comparing the suc- 

 cessive stages here represented. After the digestive cavity of the younger embryo 

 (PL II. Fig. 7) is bent at the extremities, bringing the mouth and the anus on 

 the same side of the larva, the anal and oral extremities increase rapidly in bulk, 

 and the larva, when seen from above (PL II. Fig. 18)' or in profile (PL II. Fig. 19), 

 becomes somewhat dumb-bell shaped. The depression thus formed grows deeper, 

 especially on the lower side, at the time when the chords of vibratile cilia make 

 their appearance (PL II. Fig. 21), and the mouth (PL II. Fig. 21, m) is placed in the 

 convexity of a deep curve. As the oral and anal vibratile chords extend towards 

 the oral extremity, slight grooves arise (PL II. Fig. 23), starting from the depres- 

 sion in which the mouth is placed, and extending towards the oral extremity. 

 These grooves are gouged out from the oral extremity ; they extend but little way 

 towards the stomach, forming a very well-marked channel separating the anal from 



