20 EMBRYOLOGY OF THE STARFISH. Part I. 



and another pair of smaller brackiolar arms (//), connected with the oral ventral 

 pair (e 5 e 5 ) of arms (PL III. Fig. 11). The brachiolar arms are provided at their 

 extremity with wart-like appendages (PI. IV. Figs. 4, 5, 6 ; PI. VII. Fig. 8) ; the 

 other arms have nothing of the sort, but are surrounded by chords of vibratile cilia, 

 making a complete circuit from the anal extremity round the dorsal side, while on 

 the oral side it is not closed. 



Development of the Anns. — In adult larvse the arms have, at their extremity, clus- 

 ters of orange pigment cells. These colored cells make their appearance early in the 

 younger stages, and it is easy to trace the first appearance of the arms by the 

 presence of these pigment cells. Before the appearance of the arms, the course of 

 the chord of vibratile cilia is very sharply defined ; it is like a narrow binding ex- 

 tending round the outline of the larva, seen either from above or from below (PI. 

 III. Figs. 3, 4, 6, and PL II. Figs. 26, 28). When seen in profile (PL III. v, v, Figs. 

 2, 5, 7, and PL II. v, v. Figs. 25, 27), it follows the two edges of the deep groove 

 which separates the dorsal from the ventral side. The median anal [e e) arms are 

 the first to make their appearance (PL III. Figs. 2, 3, 4, 6, 7) ; these arms take the 

 greatest development in the adult larvae ; the other arms appear also at the same 

 time, but as simple bulgings of the ciliary chord. The anal ventral pair (d" e'") and 

 the odd dorsal arm (e 6 ) are both developed about the same time (PL III. Figs. 8, 9, e e ) ; 

 the odd anterior arm increasing in size, and changing its shape more rapidly at first 

 than the median anal pair. The next set of arms formed is the dorsal pair (<?" e") ; 

 then -follows the oral dorsal pair (e'" e'"), and next the ventral oral pair (<? 5 e 5 ). 

 These develop very rapidly, and soon attain as large a size as the dorsal oral pair, 

 which had preceded them (PL III. Fig. 10). In this same figure we see the first 

 trace of a small thick arm (/"), cut off square at the extremity, placed at the 

 base of the odd anterior arm (e 6 ), and also a similar arm (//) at the base of each 

 of the ventral oral pair (e 5 e 5 ) ; the water-system branches into this small pair of 

 arms which are not surrounded with vibratile cilia (PL III. Figs. 9, 10, 11). Of the 

 brachiolar arms, the one which is odd precedes the two that form a pair. 



The chord of vibratile cilia keeps 'pace with the growth of the arms, and extends 

 to their very extremity ; the most important change which takes place, from the 

 time when the median arms first appear, is the extraordinary increase of one of the 

 diameters of the water-tubes. The portions (w, iv) extending along the stomach 

 become much flattened; when viewed from above (PL III. Figs. 8, 10, 11), their great 

 increase in size is not seen ; and it is only when examined in profile that the 

 changes the water-system has undergone in the vertical diameter, compared to the 

 transverse, can best be appreciated (PL III. Figs. 9, 12, w). 



It is in this condition that Midler has seen the greatest number of his larvae ; 

 struck by their symmetry, he has, throughout his memoirs, insisted upon the bilateral 



