NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 12/ 



Early in the evolution of the common ancestors of the Onychophora 

 and Arthropoda, judging from the embryonic development of modern 

 Onychophora, the coelomic cavities were diflferentiated into dorsal 

 compartments (fig. 34 B, a) containing the proliferation centers of 

 the germ cells in their walls, and into ventral compartments (b) open- 

 ing through the coelomoducts (c, d). With the complete separation 

 of the two series of compartments (C), the dorsal compartments 

 became gonadial sacs {G) and the ventral compartments (&) became 

 nephridial sacs. The gonadial sacs, being deprived of outlets, united 

 with one another on each side and formed a pair of longitudinal 

 gonadial tubes (E, G), which retained exit passages through one 

 pair of coelomic sacs that maintained their integrity and served as 

 genital ducts. The ventral sacs and their respective coelomoducts 

 were transformed into specific segmental excretory organs, or ne- 

 phridia. It is thus clear that the genital ducts are not "modified 

 nephridia," as they are often said to be, but that the genital ducts and 

 the nephridia are separate products of the primitive open coelomic 

 sacs, and hence, when once individually established, cannot be inter- 

 changeable in function. However, because of the variable position of 

 the genital ducts in the Arthropoda, it is evident that a dififerent pair 

 of coelomic sacs has been retained in different groups to serve as 

 genital outlets. 



Excretory organs of coelomic origin in the Arthropoda are repre- 

 sented by the coxal glands of Chelicerata and the nephridial head 

 glands of Crustacea, and perhaps also by certain head glands of 

 Diplopoda, Chilopoda, and apterygote Hexapoda. The coxal glands 

 of the Chelicerata, with one exception, consist of a single pair of 

 excretory organs situated in the prosoma and opening at the bases 

 of the appendages. Each gland in its fullest development is a com- 

 posite structure composed of several lobes or saccules derived from 

 coelomic sacs and united upon a common tubular base, the so-called 

 "stolon," or "labyrinth," composed of glandular cells and tubules, and 

 is connected with the exterior by one or two segmental ducts. The 

 organ is, therefore, variable in features that might be supposed to 

 vary in a composite structure of such a nature, as in the number of 

 coelomic sacs involved, the number of segmental openings, and the 

 position of the openings. The excretory head glands of Crustacea 

 include a pair of antennal glands ("green glands") and a pair of 

 maxillary glands ("shell glands"). The first are present in the adult 

 stage only in the Malacostraca ; the second occur in the Entomostraca 

 and in some Malacostraca ; both pairs are present in Nebalia (Manton, 



