24 EMBRYOLOGICAL MONOGRAPHS. 
ing. The slight lobes along the course of the vibratile cord indicate plainly the position of the median 
arms (e/), of the dorsal anal (e!’), the ventral anal (e//’), and the dorsal oral arms (e!/’). The greatest thick- 
ening of the vibratile cord is found at the rudimentary median arms. Beyond the mouth is shown the great 
development which the oral portion of the water-tube has taken. This and the preceding figure also show 
the mode of formation of the oral pair of ventral arms (¢5), as well as the first sign of the odd brachiolar 
appendage (7). 
31. Older larva, seen from the mouth side. Thus far the arms have altered but little the character of the outline 
of the larva. In this figure, however, some of them are sufliciently developed to be capable of considerable 
motion. The median arms (e/) especially are far in advance of the others. ‘The anal arms all develop so 
as to become more slender at first, and assume their true character earlier than the oral arms, which during 
the early stages are always more lobe-like, and take their final shape later than the anal arms. At the 
angle where the oral ventral arms and the odd arm come together, at the base of the oral arms, slight swell- 
ings are formed (/), which are the first indication of the pair of brachiolar arms (ff) ; the odd brachiolar 
arm (j"’) can only be seen in a profile view (see figs. 29, 32, 33), though in this figure it can be traced as a 
double outline of the odd arm ¢® (7). We can already see a constriction of the water-tube as it passes 
into the odd arm, and from this (nearer the mouth) are sent off two small pouches (/7 7’), (see also figs. 32, 
33,) which enter into the brachiolar pair of arms (f). The first trace of the actinal area of the future Star- 
fish is also plainly visible (¢) on the water-tube (w’) on the left side of this figure. 
32. Fig. 31 seen in profile. 
33. An adult larva seen from the right actinal profile ; the arms are in the position which they take when the 
larva is moving rapidly, arched towards the median arms, the brachiolarian arms alone being curved in the 
opposite direction from the others. In this figure the crescent-shaped ambulacral pentagon, as well as 
the lobed pentagonal outline of the abactinal area, is plainly seen. 
34. A greatly magnified figure of a full-grown Brachiolaria, at rest, in its natural attitude, with the Starfish almost 
ready to resorb the larva ; the obliquity of the planes in which the actinal and abactinal pentagons are 
situated is especially well seen in the pointed anal extremity of this Brachiolaria. No letters have been 
added to this figure, as the different parts can be readily distinguished by comparing it with figs. 31-33. 
