24 EMBEYOLOGY OF THE STAEFISH. 



Fir/s. 2, 5, 7, and PI. II. v, v, Fkjs. 25, 27), it follows the two edges of 

 the deep groove which separates the dorsal from the ventral side. The 

 median anal arms (e' c') are the first to make their appearance (PI. III. 

 Figs. 2, 3, 4, 6, 7) ; these arms take the greatest development in the 

 adult larvjB; the other arms appear also at the same time, but as simple 

 bulgings of the ciliary chord. The anal ventral pair (e'" e'") and the 

 odd dorsal arm (e*) are both developed about the same time (PI. III. 

 F'kjs. 8, 9, 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 (c" c") ; then follows the oral dorsal pair 

 (e'" e'"), and next the ventral oral pair {e" &). These develop very rap- 

 idly, and soon attain as large a size as the dorsal oral pair, which had 

 preceded them (PI. III. Ficj. 10). In this same figure we see the first 

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

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

 the base of each of the ventral oral pair (e^ e*) ; the water-system branches 

 into this small pair of arms which are not surrounded with vibratile cilia 

 (PI. 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 Avhich 

 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) extending along the stomach become much flattened ; 

 when viewed from above (PI. 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 (PI. III. Figs. 9, 12, ?c). 



It is in this condition that Miiller has seen the greatest number of his 

 larvse ; struck by their symmetry, he has, throughout his memoirs, in- 

 sisted upon the bilateral symmetry of the Echinoderm larva, as contrast- 

 ing directly wich the radiate structure of the adult animals. It appears 

 to me that this interpretation of the form of the larvae of Echinoderms is 

 incorrect ; they are radiate animals, and are no more bilateral than a large 

 number of Radiates exhibiting, as will be shown hereafter, bilateral char- 

 acters, such as Arachnactis, the Ctenophorai, the Spatangoids, and the 

 Holothurians. 



