22 EMBEYOLOGY OF THE STAEFISH. 



acter of short arms. The transformation from the pear-shaped (PI. III. 

 Fig. 1) to the rectangular flattened larva, with undulating outline (PI. III. 

 Fiy. 6), can readily be traced by comparing the successive stages here 

 represented. After the digestive cavity of the younger embryo (PI. 11. 

 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 (PI. II. Fig. 18) or in profile 

 (PI. 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 (PI. IT. Fig. 21), and 

 the mouth (PI. 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 (PI. II. Fig. 23), starting from the depression 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 the oral vibratile chord (PI. II. Figs. 25, 27). The oral is 

 less broad than the anal plastron ; the former retains its shield-like shape, 

 while the sides of the latter become somewhat undulating from the bend- 

 ing of the ciliary chord (PI. II. Figs. 26, 28). These slight imdulations, 

 as the larva grows older and more elongated, increase in size, giving it 

 more and more a rectangular outline (PI. II. Figs. 27, 28 ; PI. III. Figs. 3, 

 4). With its quadrangular shape, the larva assumes also a more flattened 

 chai'acter, and loses its cylindrical form, as will be readily seen on com- 

 paring Figs. 21 and 27, of PI. II. These slight undulations of the ciliary 

 choi-d are formed at points where accumulations of pigment cells have 

 taken place. The ciliary chord, at first simply a wavy line (PI. III. Fig. 

 4), soon becomes quite deeply indented by the formation of loops at these 

 indentations (PI. III. Fig. 6). The loops, at first, scarcely project beyond 

 the general outline of the larva (PI. III. Figs. 6, 7). Little by little they 

 increase in length (PI. III. Figs. 8, 9), extending slightly beyond the edge 

 of the outline, like short arms ; until, passing through somewhat older 

 stages (PI. III. Fig. 10), these loops are gradually transformed into larger 

 and larger arms (PL III. Figs. 11, 12), and finally attain the shape of the 

 long, slender arms of the adult Brachiolaria (PI. IV. Figs. 1, 2, 4 ; PI. 

 VII. Fig. 8). During the process of the formation of the arms, the cut 

 in which the mouth is placed becomes deeper (PI. II. Figs. 25, 27 ; PI. 



