INVERTEBRATA. 



325 



solid particles are carried on to be digested. But closer 

 examination will show, beneath this superficial resemblance, 

 very deep divergences of structure. Resemblances of this 

 kind, due to adaptation of primarily 

 distinct organisations to similar modes 

 of life, are said to show homoplasy, 

 in contradistinction to resemblances 

 such as we have found between the 

 dogfish and frog, for example, where 

 primarily similar organs have been 

 adapted to different modes of life. 

 Homoplasy may be defined as the 

 resemblance between two animals due 

 to a similar combination of analogous 

 but not homologous structures. 



Fig. 166. VIEW OF POS- 

 TERIOR END OF ANODONTA. 



(After Howes.) 



2. Shell and Mantle. The body 

 of Anodonta is always for the most 

 part hidden, and can be entirely 

 hidden, in a shell. This shell repre- 

 sents a portion of the cuticle which the epidermis (as 

 in the earthworm) everywhere secretes a portion which 

 has been greatly thickened and rendered rigid by im- 

 pregnation with calcium carbonate. The shell consists 

 of two valves, occupying symmetrical right and left 

 positions, ending along the anterior, posterior, and ventral 

 edges in an apparently free margin (though really con- 

 tinued over the rest of the epidermal surface as a delicate 

 cuticle) and united along the dorsal margin by a leathery, 

 elastic portion of the cuticle (called the elastic ligament), 

 which tends, by its elasticity, to make the valves diverge 

 from one another (fig. 165). While the animal is alive this 

 tendency is counteracted, partly or fully, by the contraction 

 of two adductor muscles which run from one valve to the 

 other and keep the ligament in a state of tension. After 

 death, when these muscles relax, the valves gape apart. 



The shell is secreted for the most part by the epidermis 

 of a pair of muscular folds constituting the mantle, which 

 grow down from the dorso-lateral region of the body and 

 cover in the sides and ventral region of the animal, some- 



