1. This is the first time that an Enteropneust with a free pharynx has been studied in the living condition.
2. The Ptychodera flava of Eschscholtz (char. emend. mihi) is rightly assigned by Spengel to his amended genus Ptychodera, as shown by the presence of the genital pleura, of external liver saccules, and by the length of the collar region.
3. P. flava belongs to Spengel's sub-genus Chlamydothorax, as shown by the ventral origin of the genital pleura, the diffuse gonads, and the free pharynx.
4. In the fact of the gill-slits being open directly to the exterior throughout their entire length, P. flava is more closely related to P. bahamensis than to any other described species. This is also indicated by the simple rows of paired liver saccules as opposed to the irregular multiple arrangement met with in P. erythræa.
5. The genus Ptychodera. (referring more especially to the sub-genus Chlamydothorax) probably represents an archaic type, as shown by the diffuse arrangement of the gonads, the free pharynx, and its littoral habitat; and it is probably not, as Spengel supposes it to be, phylogenetically younger than the other genera of Enteropneusta.
6. The gill-slits, branchial skeleton, and the temporary atrium formed by the apposition of the genital pleura in Ptychodera, offer a general homology to the corresponding structures in Amphioxus and the Ascidians, while presenting many differences in the details of their structure and relations.
7. Some of these differences are comparatively unimportant, and such as might well be expected to occur in distantly related forms with such totally different habits of existence, while others are to be accounted for by a wide interpretation of the principle of correlation between structure and function.
8. Many differences of detailed structure in the pharyngeal wall and its skeletal supports between the Enteropneusta and Amphioxus are to be correlated with the fact that, in the former, the tongue-bars are larger (often, as in P. flava, very much larger) than the primary bars, while in the latter the reverse condition obtains.