CHAETODERMA BACILLUM. 203 



fashioned into a few rather ill-defined folds, is surrounded by the customary 

 muscle sheath in which a relatively few gland cells find lodgement. These last 

 named elements are arranged in small lobules, and the component cells are 

 occasionally charged with considerable quantities of a darkly staining secretion, 

 though this is much more often greatly limited in quantity. 



Immediately anterior to the radula a prominent fold appears in the mid 

 ventral Une of the digestive tract that is probably a subradular organ. The 

 cells, unHke those elsewhere in the neighborhood, are sharply defined externally 

 and are separated here and there by ductules from two masses of gland cells 

 situated immediately ventral to the ridge in question. That the organ is 

 sensory is evidenced by the fact that it rests upon a gangUon resembling in 

 every important respect the one found in Chaetoderma attenuata for example. 



The radula (Plate 10, fig. 9) comprises the single tooth, relatively heavy 

 in this species, and the wing-like supports together with matrix cells and mus- 

 cular attachments that follow closely the arrangement of these organs in other 

 species of the genus. 



Posterior to the radula the digestive canal narrows rapidly, becomes cir- 

 cular in outline and after a brief course posteriorly unites with a more expanded 

 portion, probably the commencement of the endodermal section. This enlarged 

 division rapidly develops a variable number of small longitudinal folds — from 

 18 to 25 — that as the gut gradually narrows upon approaching the metathorax 

 (Plate 10, fig. 12), become correspondingly reduced, finally cUsappearing alto- 

 gether. The cells throughout are clearly defined elements with spherical nuclei, 

 and especially on the dorsal side beneath the aorta are distinctly glandular. 

 This state of affairs continues for at least 1.5 mm. into the metathorax. Beyond 

 this point sections were not made of the central portion of the body. 



In the posterior end of the animal the intestine, somewhat larger in cross 

 section than common, maintains the usual relations and opens into the cloaca. 

 Its cells, in favorable situations, appear to be more cubical than those of the 

 anterior end of the mid-gut, and present a denser appearance owing to a finely 

 granular secretion they enclose. Diatoms and other substances, organic and 

 inorganic, are present in abundance. 



The circulatory system is typical in all essential particulars. The heart 

 (Plate 10, fig. 14) is distinctly more muscular than the average, and the dorsal 

 aorta is not only larger than usual but it is lined with an endotheUum uncom- 

 monly distinct especially in the anterior end of the body. Beyond this point 

 no other distinctive features have been recognized. 



