120 STliorilOMKXlA TI{IAX(;iLAIUS. 



small though considerably larf^or than in the case of others, apparently younger 

 and more deeply iml)edded in the eutiele. In almost every case the 10-15 cells 

 comprising it are contracted into a mass in contact with the stalk that is very 

 sl(-nder and rarely contains more tlian two nuclei. 



The sensory atrium holds the usual position (Plate 36, fig. 0) and contains 

 tlie characteristic elements, of whicli the externa! ridge entirely surrounds the 

 atrial cavity save jiosteriorly where it joins the internal ridge. This last named 

 organ is likewise continuous across the mid line as a low inconspicuous elevation 

 wiiich more jiosterioriy hecomes developed into a very sharply defined structure 

 uniting with the outer ridge. The included area is beset with slender cirri, 

 united into grou|)s of 3-5 and composed of small cubical cells containing the 

 usual yellowish pigment. In many cases muscle fibres pass up into the central 

 cavity of each cirrus, and nerve fibres from neighboring ganglia may be traced 

 to the basal ])()rtion. The opening from the atrium into the next section of 

 tiie digestive tract- is on the posterior atrial wall and leads into a relatively long 

 l)haryngeal tube develojied internally into several longitudinal folds lined with a 

 delicate cuticle and composed of slender colunniar cells usually filled with some 

 glandular secretion, especially in the section next to the atrium. In this same 

 fourth groups of cells (shown against under surface of digestive tract, Plate 18, 

 fig. 0) appear in each section attached to the outer surface of what is ]irobably 

 the buccal epithelium. Directly opposite the outlet of the antei'ior pedal gland 

 these elements, arc in large measure replaced by others, likewise in groups, 

 and filled with a darkly staining granular secretion or more posteiiorly where 

 they are larger, with a highly vacuolated substance but little affected by haema- 

 loxylin. These glands extend backward to the radula or at the point where 

 the ventral salivary glands open. These last named organs are tubular, at 

 least 1.5 mm. long and 0.15 mm. in average diam(>ter, and open into depressions 

 on the pharyngeal wall on each side of the radula (Plate 18, fig. 9). A thin 

 epithelial lining borders the lumen while the outer surface is in contact with 

 gland cells, pyriform and filled with a secretion difi'ering considerably in different 

 specimens and parts of the same gland. .\t some points the cells are closely 

 packed with a bluisli or innkish secretion or at others this material is collected 

 into roundi'd i)articles, dark Itrownish yellow in color, surrounded by a vacuole 

 of considei-able size. 



The radula, typically located, seemingly belongs to the distichous type, 

 and yet is r(>adily related to the polystichous form occurring in other genera 

 if we assume that the bases of the once distinct teeth have secondarilv fused. 



