MUSSELS 



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exactly like the wings of insects. For this reason the fishermen of the 

 Mediterranean call these molluscs by the descriptive name of " sea- 

 butterflies." Many of these delicate creatures occur in enormous 

 swarms, and hence form an important food-supply for many higher 

 animals, especially fishes. With one species, Clio borealis, we have 

 already become acquainted as forming the staple article of diet of the 

 gigantic whalebone whales. 



CLASS II.: BIVALVES (LAMELLIBRANCHIATA). 



Bilatbeally symmetrical molluscs with bivalve shells, head absent and 

 wedge-shaped foot. 



Fresh- Water or Painter's Mussel (Unio pictorum) . 



(Length up to 3i- inches.) 



A. Structural Topography. 



This mussel may be compared to a 

 book. The two valves of the shell, which 

 are movably united on the dorsal margin, 

 correspond to the cardboard covers. The 

 first and last leaves are represented by 

 the cutaneous mantle lobes which lie 

 closely adjacent to the valves of the 

 shell (the whole inner space of the 

 mussel between the mantle lobes being, 

 accordingly, the mantle cavity). The 

 second and third leaves from each end 

 of the book are represented by the leaf- 

 like branchice, and all the remaining 

 leaves of the book taken together re- 

 present the body and foot of the mollusc. 

 The fresh-water mussel is — 



B. A Sessile Aquatic Animal. 

 In order to' study its structure and 

 habits, we place some specimens with 

 water in a dish, the bottom of which is 

 covered with gravel. (They may be 

 found in any river. They were called 

 painter's mussels because formerly their 

 greenish shells were used for holding painters' colours.) 



Transverse Section thkotjgh a 

 Lamellibp,anch (Diagrammatic) 



F., Foot ; R., body ; the white spots 

 in the body (D.) represent the in- 

 testine in transverse section ; Mt., 

 mantle ; Mh., mantle cavity ; Sch., 

 shell ; Sb. , hinge-line ; K. , bran- 

 chiie. 



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