DEVELOPMENT OF THE ARMS. 23 



HI. Figs. 2, 5, 7, 9, 12 ; PI. IV. Fir/. 4), as well as the groove extending 

 along the sides of the larva, which runs from the median anal arms (<?') 

 to the oral extremity, and separates the anal from the oral plastron. In 

 all these larvo3 the ventral part of the anal and tlie oral plastron are 

 much narrower than the dorsal portion of the anal plastron. This difler- 

 ence is at first slight (PI. II. Figs. 20, 28; PI. III. Figs. 3, 4); it hecome.s 

 more marked with advancing age, passing through the different stages 

 represented in PI. III. Figs. 6, 8, 10, 11); PI. IV. Figs. 1, 2; PI. ^11. Fig. 

 8 ; and in proportion as all the ridges and edges are more prominent, 

 the surfaces circumscribed by them become flattened and more spreading. 



Nomenclature of the Arms. — For the sake of brevity, I shall call the rudi- 

 mentary appendages by the names proposed for them in the adult lar\'ae, 

 and shall adopt the names given by Miiller, with sliglit modifications, viz. 

 ventral side, that on which the mouth is situated; dorsal, the side on 

 which the water-pore is placed ; anal plastron, what Miiller has called 

 "anales Bauchfeld," or " hinteres Bauchfeld", oral plastron, what he 

 calls "antorales Feld," or "vorderes Bauchfeld"; the oral region (m) is 

 situated between these two plastrons. The arms are designated accord- 

 ing to their position by the following names : the median anal pair 

 (/ e); the dorsal anal pair (e" e"); the ventral anal pair (e" e"); the 

 dorsal oral pair (e"" e'") ; the ventral oral pair (e^ e^) ; the odd anterior 

 arm [e^), from which projects, at the base, a single arm of a different char- 

 acter from the others; the odd brachiolar arm (/'); and another pair 

 of smaller brachiolar arms (//), connected with the oral ventral pair 

 {e^ e^) of arms (PI. 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 extrem- 

 ity round the dorsal side, while on the oral side it is not closed. 



Devehpment of the Arms. — In adult larvjB the arms have, at their ex- 

 tremity, clusters of orange pigment cells. These colored cells make tlieir 

 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 extending round the 

 outline of the larva, seen either from above or from below (PI. III. Figs. 

 3, 4, 6, and PI. II. Figs. 26, 28). When seen in profile (PI. 111. v, v\ 



