Stereobalanus canadensis, a species with long slit-like external gill-pores, is interesting in possessing a well-developed genital wing both dorsally and ventrally to the series of gill-pores of each side.

Each reproductive gland opens by its own pore or pores directly to the exterior. Several glands and pores may occur in the same transverse section.

According to Spengel there is no definite relation between the number of the reproductive organs and that of either the gill-sacs or the liver-outgrowths. The only definite segmentation exhibited by Balanoglossus is thus the division into three regions which is so distinctly shown by the arrangement of the body-cavities; though the gill-sacs may indicate an incipient further segmentation of the major part of the body. In this connexion it is interesting to notice MacBride's statement[[22]] that the body-cavity of Amphioxus develops in the embryo as five cavities, just as in Balanoglossus; the segmented part of the body being formed by a secondary segmentation of the third body-cavities.

Regeneration.—Balanoglossus, like Phoronis (p. [30]), possesses great powers of regenerating lost parts. The posterior part of the body is readily re-formed, while Spengel has shown[[23]] that even the proboscis, collar and branchial region can be regenerated, apparently from a fragment of the body.

Genera of Enteropneusta.—Spengel, whose Monograph is indispensable to every student of the Enteropneusta, formerly proposed to divide the old genus Balanoglossus into four; but he now recognises no less than nine.[[24]] Some of the more important characters are given below, but for the arrangement of the muscles, important from a systematic point of view, reference must be made to the original sources.

A. Notochord with a vermiform process (Fig. 7, v); pericardium with anterior diverticula more or less developed. .......... Glandicipitidae

(a) Liver-sacs and synapticula present; gill-slits almost equalling the pharynx in depth, so that the ventral, non-branchial part of the pharynx is reduced to a mere groove (Fig. 6); nerve-roots absent; pericardial diverticula long. .......... Schizocardium, Speng.

(b) Liver-sacs absent;[[25]] ventral part of pharynx well developed; pericardial diverticula short.

(i.) Synapticula and nerve-roots absent. .......... Glandiceps, Speng.

(ii.) Synapticula present; nerve-roots present or absent; genital region with dermal pits. .......... Spengelia, Willey.