NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SXODGRASS I7 



the embryo, and the enterocoele theory has some plausibility as a 

 wide generalization, considering the very common early association 

 of the coelomic mesoderm rudiments with the endoderm ; but, as 

 applied to the annelids and arthropods, the theory must entirely discard 

 the direct evidence from embryology that the mesoderm first appears 

 as solid proliferations of cells, which only in a purely hypothetical 

 manner could be interpreted as representing pouches of the archen- 

 teron. The only known case of the formation of the mesoderm from 

 enteric pouches that might be referred to the articulates occurs in 

 the Tardigrada (see Marcus, 1929), but there is much uncertainty 

 concerning the relationships of the tardigrades. 



A second mesoderm theory is the gonococle theory, based on the 

 almost universal association of the germ cells with the coelomic meso- 

 derm in the coelomate animals. Hatschek (1877, 1894) believed that 

 the mesodermal teloblasts of the annelid larva are themselves germ 

 cells, and Rabl (1879, 1889) adopted this view. The gonocoele theory 

 of the origin of the coelomic sacs, however, was principally elaborated 

 by E. Meyer (1891, 1901). Meyer contended that the primitive 

 coelomic sacs were muscular pouches, from the epithelial walls of 

 which the germ cells are generated, and that, as these gonadial sacs 

 expanded to increase the reproductive function, they finally preempted 

 the haemocoele, and their muscles were transferred to the body wall. 

 The gonocoele theory loses much of its support now that the old belief 

 that the germ cells are direct products of the coelomic epithelium 

 is no longer tenable, and, moreover, it entirely breaks down in view 

 of the fact that the primary larval somites of the annelids do not 

 contain germ cells. In the primitive annelids, as will be shown later, 

 the germ cells probably were located in the zone of undifferentiated 

 tissue behind the last primary somite. If so, the reproductive function 

 had nothing to do with the origin of the mesoderm or the formation 

 of the coelomic sacs. 



A third theory, concerned principally with the function of the 

 coelomic sacs, is the nephrocoele theory (Ziegler, 1898; Faussek, 1899, 

 1901). According to Ziegler, the primitive coelomic cavities were 

 open pouches for the accumulation of waste products ; they were not 

 diverticula of the archenteron, but were, perhaps, of the nature of 

 protonephridia. The nephrocoele theory as modified by Faussek holds 

 that the excretory coelomic sacs are not primitive structures in a 

 phylogenetic sense, but that they have been developed for excretory 

 purposes in the embryo, and are hence purely ontogenetic organs. 

 Faussek supports his theory with the generalization that the open 

 metanephridia constitute exits from the coelom, while the closed 



