114 STKOPHOMKNIA OIMIIDIANA. 



it demands no doscription. Posteriorly the gut narrows rapidly, becomes some- 

 what rectangular in cross section as it passes between the limbs of the shell 

 gland, and shortly before its outlet in the cloaca develops moderately high folds. 

 The pericardial cavity is of very large size (Plate 18, figs. 2, 3), and the con- 

 tained heart is of the usual greatly elongated type. There are no distinct signs 

 of a division into uurich- and ventricle though a valve-like flap near its anterior 

 end may indicate such or possibly the commencement of the aorta, which for a 

 considerable distance is of as great diameter as the heart itself and even in the 

 head region <H)ntinues of large calibre (Plate 18, fig. 1) • Its relations to the gonad 

 and visceral cavity are similar to what occurs in S. triangulnris. In the pos- 

 terior part of the body the channels are more than usually ill defined, but the 

 course of the blood is essentially the same as in the other species of the genus. 

 The corpuscles possess the characteristic elliptical or pointed ovate form, and 

 are accompanied by a relatively large number of leucocytes. 



The gonad is fully developed, of relatively large size and the sex products 

 are arranged in the customary fashion. Throughout its entire extent, Init 

 especially in the posterior half of the animal, the normal reproductive elements 

 are associated with large masses of eggs in all stages of degeneration. This may 

 be due to post mortem changes, but the sharply defined character of the various 

 stages of the spermatozoa, ova, blood corpuscles, and other cellular elements 

 in various parts of the body argues against such a view. In some species of 

 Chitons (e. g. Ischnochiton 'magdalenensis) a considerable number of ova do not 

 pass to the exterior during the egg-laying process, but undergo disintegration 

 and are absorbed. Appearances indicate that this is the state of affairs in the 

 present species, and the almost empty condition of the seminal receptacles further 

 indicates that the breeding season has passed. 



The ducts leading from the jjericardial cavity are relatively slender though 

 they enlarge somewhat before entering the shell gland, and as the cells change 

 from a cubical to a columnar form they become increasingly glandular. An 

 unusually large number of seminal vesicles are present, twenty-three occurring 

 on the side of the body represented (Plate 9, fig. 1). In these the distal, usually 

 vesicular portion is exceptionally small (Plate 18, fig. 4) but the diameter may be 

 somewhat increased when the organs are filled with sperms. These bodies are 

 attached not only to the forward end of the shell glantl but several of them open 

 into the dorsal section of the gonoduct. The component cells are columnar and 

 show at various points faint signs of glandular activity. The shell gland on the 

 other hand is highly glandular, more than usually irregular in outline and as 



