206 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



(f) The pair of sensory patches or bspEradia, which 

 probably cover the vestiges of the ancestral pair of 

 ctenidia. Each structure is a minute orange or reddish 

 coloured elevation on the floor of the nuchal cavity close 

 against the shell muscle on either side. It is nearer the 

 back of the nuchal cavity than the tip of the shell muscle. 

 These structures prove that the nuchal cavity is really a 

 branchial cavity of which the ctenidia have degenerated. 



Body Wall, Muscular System, and Mantle. 



The body of Patella is covered by a layer of columnar 

 epithelium which in most parts possesses a well-developed 

 cuticle. That covering the visceral hump is mostly black- 

 pigmented as already mentioned ; that covering the regions 

 of muscular attachment is, as might be expected, much 

 flattened. A great deal of the epithelium is ciliated in 

 young animals. Unicellular glands are not common in 

 the epidermis, but a few occur in the foot and mantle; 

 they are of the usual goblet type. The covering tissues 

 will be discussed in greater detail in connection with the 

 various organs. 



Muscular System. — Though muscle fibres are found in 

 almost every part of the animal, there are special aggre- 

 gations of them which need separate mention. These are 

 the body-wall muscles of the head and neck, the foot, the 

 shell muscle, the pallial muscles, and the muscles of the 

 odontophore. The last of these will be described in 

 treating of the odontophore. 



The muscles of the dorsal body-wall in the head region 

 comprise the following layers: — 



(a) An outer transverse layer, (/>) a layer of longitudinal 

 and obliquely arranged fibres which are head retractors. 

 There are also some fibres running transversely internal 

 to (6). 



