OQO INTRODUCTION 
it a rudimentary, laminar, calcareous testa. The ante- 
rior part of the mantle is free and movable, and the head, 
indirectly through the retractor muscle of the buccal 
body, is capable of being retracted beneath it. On the 
right edge of the mantle the pulmonary orifice exists ; 
and at the posterior side of the latter the anal aperture 
is placed. Upon the right side of the head, a short dis- 
tance posterior to the superior tentacula of that side, the 
genital orifice is situated. The body has two distinct 
cavities ; the pulmonary chamber, containing a vascular 
net-work upon its surface, the heart, the renal organ, and 
the rectum ; and the visceral cavity, separated from the 
former by a muscular partition, containing the digestive 
and generative apparatus and the nervous centres. 
In Tebennophorua the mantle covers the whole upper 
surface of the body, and encloses no testaceous rudiment. 
Its anterior edge is unattached, and the head is retractile 
beneath it. The pulmonary chamber is placed beneath 
the anterior part of it ; and the muscular membrane 
bounding the visceral cavity in a great part of its extent, 
is but loosely attached to the outer integument. 
In Vayinula, the body appears broad from the mantle, 
which encloses the whole body except the comparatively 
narrow pedal disc, forming a lateral, angular projection 
as it is inflected inferiorly to the margin of the pedal 
disc. In transverse section it is semi-elliptical, as repre- 
sented in figure 4. The mantle contains no testaceous 
rudiment. The head can be but slightly protruded. The 
respiratory orifice is situated on the right side of the tail, 
