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tions of their flexible bodies, and rapaciously catch with their 

 elastic hooked tongues all the small pelagic animals that sur- 

 round them. 



The fin-snails have a curious shape, which differs in almost 

 all respects from that of the typical snail. The head-part is 

 only indicated by a mouth surrounded by rudimentary feelers. 

 The body, which is of various shapes, is frequently hidden 

 by a delicate shell; but the most singular organ is a pair of 

 large wing-like fins , which are situated on the head or neck, 

 and are used by the animal like the wings of a butterfly. The 

 people call this creature far folia di mare (sea-butterfly). The 

 most frequent kind is the Hyalaea, with a palebrown horny 

 covering , and large ever-moving fins. It often appears in 

 swarms, but in the Aquarium lives scarcely a day. 



The Cymbulia and Tiedemannia are much rarer. 



The Shellfish are the lowest group of mollusks. They are 

 externally distinguished from the snails by their shell, which 

 invariably consists of two moveable valves, that close by means 

 of a muscle, and open, when the muscle is relaxed, by means 

 of an elastic band on the so-called lock. The want of a head 

 is an important characteristic of these animals. The organ of 

 locomotion is a foot generally shaped like a wedge or tongue, 

 and capable of being pushed forward by swelling. The body 

 is covered by the leafy gills and mantle-plates, which secrete 

 the shell. These mantle-and gill-plates lie under the shell like 

 the leaves of a book. The internal organs are : an intestine , 

 a liver, a heart, kidneys and the organs of generation. The 

 ciliated membranes of the gills and the mantle conduct the 

 water containing air and nourishment. A nervous system and 

 organs of sense (ear-bladders and often eyes) are found in all 

 these animals. The greatest number of shell-fish beiong to 

 the sea, where they either stick immoveably to some object, 

 like oysters, or hide in the mud, and are capable only of ex- 

 tremely slow movements ; very few canswim or jump. From 

 the large number of those living in the Mediterranean we will 

 choose the most interesting and important. 



The common mussel (Mytilus edulis) has a blueblack three- 

 cornered shell with a pointed whorl. This animal possesses a 

 so-called byssus-gland, from which it spins horny threads or bys- 

 sus wherewith it fastens itself so firmly to rocks or piles that 

 the most violent breakers or currents of water are unable to 



