24 EMIiRYOLOGlCAL MONOGRAPHS. 



iiig. The slight lobes along the course of the vibratile conl iudicate plainly the position of the median 

 arms {<;'). of the dorsal anal (<;"), the ventral anal {<:'"), and the dorsal oral arms {c'"'). The greatest thick- 

 ening of the vibratile cord is found at the rudimentary median arms. Beyond the mouth is shown the gi-eat 

 development whiidi 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 (t^), as well as the first sign of the odd brachiolar 

 appendage (/')• 

 31. Older larva, seen from the mouth side. Thus far the arms hare altered but little the character of the outline 

 of the larva. In this figure, however, some of them are sufficiently developed to be capable of considerable 

 motion. The median arms (c') especially are far in advance of the others. The anal arms all develop so 

 as to become more slender at first, and a.ssume 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 ami 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 (//") ; the odd brachiolar 

 arm (/') 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 ami o^ (/'')• ^^'^ <^'^" already see a constriction of the water-tube as it passes 

 into the odd arm, and from tliis (nearer the mouth) are sent off two small pouches (/'./'), (see also figs. 32, 

 33,) which enter into the brachiolar pair of arms (/). The fii'st 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 amhulacral pentagon, as well as 

 the lobed pentagonal outline of the abactinal area, is plainly seen. 



34. A greatly magnilied figure of a full-grown Braehiolaria, 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 Braehiolaria. No letters have been 

 added to this figure, as the different parts can be readily distinguished by comparing it with figs. 31-33. 



