ALEXANDROMENIA VALIDA. 145 



in the foregoing species. The smallest dorsal glands (dsg) are more or less im- 

 bedded in the pharyngeal wall from the cirrose cavity to the posterior end of the 

 radula sac. Each consists of an aggregation of well-marked pyriform cells 

 usually charged with a finely granular darkly staining secretion. In some of 

 the larger groups the secretory jiroducts are not so clearly granular and have a 

 more reddish cast, in this respect and in general appearance resembling the second 

 type of dorsal salivary gland. 



The second species of gland (sg) is in reality a paired structure each half 

 consisting of about two dozen lobulate glandular bodies united by as many 

 branches of a main duct which opens into the pharyngeal cavity. It appears 

 probable that each organ arose in the embryo as a diverticulum of the gut, 

 and subsequently developed outgrowths in which some of the cells became 

 glandular. These retaining their connection with the lumen of the duct 

 elongated greatly, became pyriform and formed the lobule of the completed 

 gland. As in A. agassizi the canal in each lobule develops small latei'al 

 branches and in any case the duct holds a superficial position. 



The course of the main duct, which lies to the outside of the glands is shown 

 in Plate 21, fig. 2. It opens at the base of a corrugated papilla enclosed in a 

 diverticulum of the pharynx that in turn opens at the forward border of a broad 

 papilla on the pharyngeal wall (Plate 11, fig. 3). As in the foregoing species the 

 papilla contains a few small canals which open on its surface, but their inner con- 

 nections are difficult to trace. They appear to be the outlets of a number of 

 small glands belonging to the first type which, as noted above, approach the 

 second in the form of the cells and the character of their secretion. 



The tubules of the third set (gl) are in form and position like those of A. 

 agassizi. The secretion is more abundant and more granular and darkly 

 staining, giving them a denser, more compact appearance, but this set is not 

 voluminous as in the preceding species. 



The radula is of the distichous type (Plate 11, fig. 3, Plate 34, fig. 14) and 

 contains approximately thirty-four rows of teeth. These are developed by lArge 

 numbers of exceedingly slender odontoblasts, and immediately after their forma- 

 tion are enveloped in sheaths composed of numerous so-called enamel cells. 

 Both of these groups blend with cells that become smaller as the opening of the 

 radula sac is approached. 



At the forward border of the radula the cells of the pharyngeal wall become 

 more columnar, less dense, and their nuclei assume a more slender shape. Ap- 

 pearances suggest a subradular sense organ, but it lacks the definiteness of this 



