224 KINGSLE Y. [Vol. V 1 1 1 . 



in Fig. 68, where also may be seen, at either side of the gill- 

 leaf larger lacunae, forming the afferent and efferent blood 

 channels of the lamella. 



In the gill appendage as in the operculum (appendage VII) 

 is a central rod of compact tissue somewhat resembling car- 

 tilage, but whose fate I have not traced. In my former 

 paper ('85, PI. XXXIX, Fig. 38) the line from the abbreviation 

 for muscle, was by mistake of the lithographer, run to this 

 structure. On either side of this rod are developed the muscles 

 of the appendages. They have their origin in a narrow line on 

 either side of the back (Fig. 6"]) and are inserted in the wall of 

 the appendage at the lower end of the rod just referred to. 

 In transverse section these muscles (Fig. 6^) are seen to be 

 fan-shaped, the line of insertion embracing nearly the entire 

 width of the appendage. From its origin and insertion the 

 muscle in front of the rod is seen to be antagonistic to that 

 behind and we may consider the two as respectively flexor and 

 extensor in function.^ The flexors of one somite and the 

 extensors of the next have their origins closely approximate, 

 and their traction soon results in the drawing inwards a small 

 patch of the dorsal ectoderm, thus producing in the adult the 

 line of depressions on either side of the middle of the tack, 

 and the corresponding chitinous ingrowths in the interior of 

 the same region. One of these ingrowths of chitin secreting 

 ectoderm is cut across in Fig. 68 ent. 



In the last stage studied (Fig. 80) the conditions are essen- 

 tially the same as before, except that the gill-leaves are larger 

 and more numerous. The section, however, does not pass in 

 the right plane to show the muscles and internal rod. For a 

 description of this section I cannot refrain from quoting Mac- 

 Leod's ('87, p. 4) description of the lung-book of a scorpion: — 

 "Nous trouvons en [la figure] la coupe d'un certain nombre de 

 fines lamelles, les lamelles pulmonaires, placees horizontalement, 

 libres a leur extremite posterieure ou caudale, c'est-a-dire la 

 plus rapprochee de 1' extremite caudal de I'animal, vers la droite 



1 I have been unable to identify with certainty these muscles with those of the 

 adult as described by Benham ('83). At this early age they are not differentiated 

 as they are later. 



