Chap. I. NOMENCLATURE OF THE ARMS. 19 



the oral vibratile chord (PI. II. Figs. 25, 27). The oral is less broad than the anal 

 plastron; the former retains its shield-like shape, while the sides of the latter become 

 somewhat undulating from the bending of the ciliary chord (PI. II. Figs. 26, 28). 

 These slight undulations, as the larva grows older and more elongated, increase in 

 size, giving it more and more a rectangular outline (PL II. Figs. 27, 28; PI. III. 

 Fig. 3, 4). With its quadrangular shape, the larva assumes also a more flattened 

 character, and loses its cylindrical form, as Avill be readily seen in comparing Figs. 21 

 and 27, of PI. II. These slight undulations of the ciliary chord are formed at points 

 where accumulations of pigment cells have taken place. The ciliary chord, at first 

 simply a wavy line (PI. III. Fig. 4), soon becomes quite deeply indented by the 

 formation of loops at these indentations (PI. III. Fig. 6). The loops, at first, 

 scarcely project beyond the general outline of the larva (PI. III. Figs. 6, 7). 

 Little by little they increase in length (PL III. Figs. 8, 9), extending slightly 

 beyond the edge of the outline, like short arms ; until, passing through somewhat 

 older stages (PL III. Fig. 10), these loops are gradually transformed into larger and 

 larger arms (PL III. Figs. 11, 12), and finally attain the shape of the long, slender 

 arms of the adult Brachiolaria (PL IV. Figs. 1, 2, 4; PL VII. Fig. 8). During 

 the process of the formation of the arms, the cut in which the mouth is placed 

 becomes deeper (PL II. Figs. 25, 27 ; PL III. Figs. 2, 5, 7, 9, 12 ; PL IV. Fig. 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 larvse, the ventral part of the anal and the oral plastron 

 are much narrower than the dorsal portion of the anal plastron. This difference 

 is at first slight (PL II. Figs. 26, 28; PL III. Figs. 3, 4); it becomes more marked 

 with advancing age, passing through the different stages represented in PL III. Figs. 

 6, 8, 10, 11 ; PL IV. Figs. 1, 2 ; PL VII. 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 rudimentary 

 appendages by the names proposed for them in the adult larvaa, and shall adopt 

 the names given by Midler, with slight modifications, viz. : ventral side, that on 

 which the mouth is situated ; dorsal, the side on which the water-pore is placed ; 

 anal plastron, what Muller has called " anales Bauchfeld," or " hinteres Bauchfeld " ; 

 oral plastron, Avhat 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 folloAving names : the median anal pair (e' e) ; the dorsal 

 anal pair (<?"/'); the ventral anal pair (e" e'"); the dorsal oral pair (e"" e""); the 

 ventral oral pair (e 5 e 5 ) ; the odd anterior arm (e 6 ), from which projects, at the base, 

 a single arm of a different character from the others ; the odd brachiolar arm (/") ; 



