8o 



SMll'HSONIAX M1SCI:LLANE0US COLI.iaTlONS 



vol.. 8 1 



merely a specially developed exite lobe of a single shaft. Borradaile 

 (191 7) expresses the opinion that " probably the primitive crustacean 

 appendage resembled that of the Branchiopoda in being uniramous." 

 Movable lobes individually provided with muscles, however, may be 

 developed along both the outer and the inner margin of the Umb, and 

 an excessive development of one of the outer lobes might give rise to 



L.: 



Fig. 33. — Parapodium and parapodial musculature of an annelid worm 

 (Nereis virens). 



A, B, first and third parapodia, left, anterior surfaces. 



C, cross section of left half of a segment from middle of body, cut anterior 

 to base of parapodium. showing muscles of setae inserted on end of setal 

 pouch (a), and ventral promotor (A.') and remotor (L) muscles of parapodium. 

 DMcl, VMcl, dorsal and ventral bands of longitudinal body muscles. 



D, musculature of third parapodium, right, inner view, showing tergal pro- 

 motor (/) and remotor (7), and sternal promotor (K) and remotor (L). 



E, musculature of right side of a segment from middle of body, internal 

 view, lateral oblique muscles and setal muscles removed : b, c, anterior and pos- 

 terior pleuro-sternal muscles ; DMcl, part of dorsal longitudinal muscles ; /, tergal 

 promotor of parapodium ; /, tergal remotor ; /, accessory remotor arising 

 anteriorly from intersegmental fold ; K, sternal promotor ; L, sternal remotor ; 

 .SV^ bases of setae. 



a secondary biramous structure of the appendage. Hansen (1925) 

 recognizes the definitive two-branched structure of the typical crus- 

 tacean appendage, but he says it seems " impossible to deny the possi- 

 bility that the exopod may be analogous with the epipod, and if so 

 the primitive appendage is uniramous." 



The segmental appendages, or parapodia, of the polychaete annelids 

 are in some cases simple lobes ; in others they are of a two-branched 



