1894 - 95 .] Dr Gilchrist on Torsion of the Molluscan Body. 367 
mantle cavity proper disappears, and the mantle projects on all 
sides, leaving a groove between it and the foot. The pallial organs 
lie in this groove on the right side, and show some important varia- 
tions in position. 
Pleurobranchus (fig. 8) may he taken as a type of this arrange- 
ment. The various pallial organs are arranged practically in a line. 
There are well developed rhinophora; no osphradium, hut in its 
place an opening (gl) which leads into a glandular sac of unknown 
function. I^ephridial opening and gill are situated alongside of 
each other, while the anus is just behind the gill. 
A closely allied form, Pleurobranchea (fig. 9), diflfers curiously in 
some details. The foot shows no broadening out as in Pleuro- 
branchus : the mantle is reduced to a mere ledge round the body, 
and the shell has been found to exist only in a very reduced condi- 
tion. The genital opening is further removed from the head region 
and the gland above mentioned; gill and nephridial opening are 
situated posteriorly, and occupy a striking position relatively to each 
other. As will be seen from the figure, the gill is now the leading 
organ in the backward retreat. It is difficult to suppose this to be 
the result of a “ favourable variation,” as it is just between two 
excretory organs. Perhaps this has something to do with the fact 
that in passing over to the Nudibranchs we find a form in which 
