200 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 Tebennoj^Jiorus 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 Vaginula, the body appears broad from the mantle, 

 which encloses the whole body except the comparatively 

 narrow pedal disc, forming a lateral, angvilar 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, 



