524 H. p. KJERSCHOW AGERSBORG 



of respiration, that is, they are at least partly respiratory in 

 function ; for this purpose large intercellular sinuses are 

 present which communicate with the heart through the efferent 

 branchial veins (PI. 30, figs. 25, OsjJ, 54, Aur). Such sinuses, 

 however, are not present in the papillae only. 



8. The Foot. 



The foot (PI. 27, fig. 1, PL 28, fig. 9) extends a little beyond the 

 trunk both anteriorly and posteriorly. Its general form is like 

 an inverted flat-bottom dory, with the anterior end wider than 

 the posterior, curved, and extended forward about 1 cm. 

 from its base, the so-called keel. The posterior end, also 

 curved but narrower than the front, projects about 1-5 cm. 

 from the base in an adult. The edges bend considerably 

 outward, making the width from rim to rim about twice 

 as wide as the base. Between the edges, on the ventral side, 

 is a depression so deep that the comparison with a dory is 

 quite fitting. This groove is highly tuberculate and ciliated 

 (PI. 30, figs. 26, 27, Cil, Thr). The internal structure of the 

 foot varies. The anterior end has a fine network of nerve-cells 

 spread throughout its length, and at the posterior end is an 

 aggregation of nerve-cells into a ganglionic centre (PI. 80, 

 figs. 26, 27, 28, 29). The anterior end of the foot has also 

 a great number of small glands which open to the outside all 

 along the foot by fine crypts through the ciliated ectoderm 

 (PI. 30, figs. 28, 29, Mug) and decrease in number and size 

 toward the posterior end. The secretion of these glands is 

 perhaps of use to the animal in helping it to move over fronds 

 of marine vegetation and other solid objects. Very fine neural 

 fibres extend from the pedal ganglion (PI. 30, fig. 29, Nt) to the 

 base of the ciliated columnar epithelial cells and to the glands. 

 This suggests that the cilia of the ectoderm of the foot and the 

 glands in the foot are under nerve control. And, as I have 

 previously recorded (1919, 1921), M. leonina may move 

 without any visible bodily contortions, the foot being then 

 either in touch with the surface tension of the w^ater. or with 

 the fronds of marine vegetation or some other solid. In the 



