14 



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

 prohahly cover the vestiges of the ancestral pair of 

 cteuidia. Each structure is a minute orange or reddish 

 cohnired elevation on the floor of the nuchal cavity close 

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

 hack of tlie nuchal cavity than tlie ti]) of the shell muscle. 

 Tliese structures prove that tlic nuclial cavity is really a 

 hranchial cavity of which the ctenidia have degenerated. 



V>^)^^Y "Wall, Mfsci'lar System, axi) 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. T^nicellular glands are not common in 

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

 they arc 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 tlie muscles of the 

 odontophore. The last of these will bo described in 

 treating of the odontophore. 



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

 comprise the following layers : — 



{(t) An outer transverse layer, {h) a layer of longitudinal 

 and obliquely ai'ranged fibres which are head retractors. 

 There are also some tibres running tiansversely internal 

 to {1>). 



