752 PROCEEDINGS OF THE ACADEMY OF [Oct,, 



onnaire" or "premiere forme larvaire/' and the immature worm found 

 as a parasite in insects the "etat larvaire proprement dit" or "deux- 

 ieme forme larvaire.'' Camerano and Vejdovsky (1894) are quite 

 right in insisting, in opposition to Villot and von Linstow, that the 

 organism as it leaves the egg-membranes is the larva, and that thefe 

 is no second larval stage. That stage is the larva which is equipped 

 with a temporary larval organ, the proboscis ; all later stages, as far as 

 they are known, are but progressions toward the adult condition. 



4. Conclusions. 



The opinion I had previously expressed (1903) from a study of the 

 adult anatomy of Paragordius was as follows: "With the Nematoda 

 the Gordiacea have in common only one important structure, the 

 tubular genitalia and their opening into the cloaca. With the Annelida 

 they agree in the structure and innervation of the musculature, and 

 in having dorso-ventral mesenteries which di\dde body-cavities 

 bounded by cell-layers. But there is good reason for doubting the 

 homologies of these mesenteries and body-cavities in these two groups 

 of animals. They differ from the Annelida in showing no trace at all 

 of true metamerism (either in the nervous system, the body-cavities, 

 the genitalia or the body wall) ; in the absence of a cerebral ganglion, 

 of a vascular system, of setal sacks or extremities ; and very markedly 

 in the structure of the genitalia. In ^-iew of these facts the Gordiacea 

 cannot be regarded as even highly degenerate Annelida, as Vejdo\'sky 

 has done, nor yet as modified Nematoda, the view of most of the 

 writers, but must rather be considered a group of isolated position, 

 as Grenadier, von Siebold and Villot have regarded them." 



The present study of the early development confirms this \aew, but 

 shows that there is considerable correspondence of the cleavage and 

 gastrulation processes wdth those of the Nematoda. The middle germ- 

 layer is not a connected mesoderm, but a mesenchym of disconnected 

 cells, arising from the mesentoderm close to the blastopore. 



The larva has two marked peculiarities. First, by a transverse 

 septum, the diaphragm, the body is partitioned into an anterior 

 proboscis and a posterior head-trunk. This proboscis is entirely ecto- 

 dermal except for mesenchymatous muscles, and lacks any nervous 

 system, excretory system, and any portion of an intestine. Though 

 it composes half the body, it is probably a purely larval organ that 

 takes no part in the adult structure : the diaphragm marks the future 

 boundary of the adult head, and the proboscis is precephahc. A sup- 

 port for this view, apart from good anatomical considerations, is to 



