210 MORPHOLOGY OF INVERTEBRATE TYPES 



The first two are produced by the edge of the mantle, while the 

 third layer is a secretion of the entire surface of the mantle. 

 In Venus mercenaria the mother-of-pearl layer is very thick 

 but does not possess the aspect of the corresponding layer in 

 other shells. The periostracum is thin and easily worn off by 

 friction against the mud and sand in which the animal lives. 

 The edge of the mantle or pallium leaves a distinct impression 

 on the inside of the shell called the pallial line. At the posterior 

 end of the shell this line shows a distinct triangular pallial 

 sinus marking the position of the retractor of the siphon. Four 

 muscles attached to the shell leave also their impressions on it. 

 These are the two powerful adductors of the valves and the two 

 small retractors of the foot. The impression of the anterior re- 

 tractor of the foot is always separate from the impression of the 

 much larger one of the anterior adductor of the valves and is 

 situated above it. The posterior retractor of the foot leaves 

 seldom, if ever, an impression of its own, separate from that of 

 the posterior adductor of the valves; usually it merges with the 

 latter, forming a common impression. 



As has been stated, the shell is produced by the mantle or 

 pallium, the latter being simply a fold of the body covering. 

 It is not extraordinary, therefore, that both surfaces of the man- 

 tle, i. e., the one underlying the shell and the other facing the 

 mantle cavity, are lined with eipthelial cells of ectodermal origin. 

 There are naturally numerous glands in the outer epithelial 

 layer of the mantle. Between the two epithelial layers are 

 connective tissue and muscular fibres. The latter form three 

 systems: (i) muscle fibres which run in the free margin of the 

 mantle at right angles to its edge, (2) fibres parallel to the edge, 

 and (3) short fibres traversing the mantle from its outer to its 

 inner surface. The first of these systems forms a regular band 

 visible with the naked eye. Near the posterior end of the body 

 the right and left mantle folds are grown together in two places, 

 one above the other, thus forming two short tubes or siphons. 

 These may be readily recognized by their dark pigmentation. 



