162 CONVERGENCE IN MINUTE STRUCTURES 



though quite different topographically, has been 

 described by M. Stenta 1 in a Protobranchiate 

 bivalve mollusc. These are the marginal pallial 

 sense-organs of Leda commutata (Nuculidae) which 

 occur right and left at the anterior junction of 

 the right and left mantle lobes, lying in a crypt 

 on each side between the inner and middle folds 

 of the pallial margin. If any one ventured to 

 argue that the posterior adrectal or abdominal 

 organs of Lamellibranchiata might be related by 

 way of homology to the segmental abdominal 

 organs of Capitellidae, the same argument could 

 not be applied to the anterior pallial organs of 

 Leda, although all these structures apparently 

 belong to one physiological category, namely, 

 rheostatic organs. We may confidently conclude 

 that they are related to each other only by way 

 of sensory convergence. In this connection we 

 may take note of the well-known case of retinal 

 convergence between the pallial eyes of the scallop 

 and the cerebral eyes of vertebrates. 



Eisig 2 treated the lateral sense-organs of Capi 

 tellidae under two categories thoracic organs 

 appearing in surface view as open pores ; and 

 abdominal organs, appearing as retractile knobs or 

 elevations, the distal extremity of which is beset 

 with stiff sense - hairs radiating in all directions. 



1 Mario Stenta, "Uber ein neues Mantelrandorgan bei Leda 

 commutata? ZooL Anz., xxxv., 1909, pp. 154-157. 



2 Hugo Eisig, " Die Capitelliden." " Fauna und Flora des Golfes 

 von Neapel." Berlin, 1887. 



