THE PTEROPODA. 



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The Sea Lizard is very common in many parts of the Atlantic, where it is found in 

 vast numbers during a calm and when the sea is smooth. Mr. F. D. Bennett writes as 

 follows about this strange and eccentrically formed being. " These creatures obtain 

 in greatest number where currents most prevail ; they are active and very predatory 

 in their habits, and would appear, from the observations of my brother, which I have 

 already confirmed, to subsist chiefly upon the soft parts of the defenceless genera 

 Velella and Porpita. The specimens we captured and kept in sea-water contracted their 

 bodies into many convulsive attitudes, but seldom employed their branchial fins, and 

 floated buoyantly while passive. When immersed in fresh water they contracted them- 

 selves into a very small compass, assumed a globular form, cast the tentacles from off 

 their branchial fins, lost their color, and expired in a very few moments. 



When handled, the white coloring matter of the body is easily detached, and leaves 

 the corresponding portions of the integument transparent ; when a considerable accum- 

 ulation of air, in the form of small bubbles, is visible among the viscera, and fully 

 accounts for the hydrostatic power the animal possesses. The color of the entire upper 

 surface of the body is dark blue, with the exception of a broad silvery streak extending 

 from the head to the extremity of the tail ; the inferior surface of the body and fins 

 is pearl-white. The head also is white, but has a vertical blue line on its centre. The 

 entire animal is enclosed in a tough, but smooth and delicate integument. 



WE now arrive at a new order of molluscs, if possible stranger than that which has 

 just been briefly described. The animals of this order are inhabitants of the sea, but 

 differ from their kind in living almost wholly on the surface of the waters instead of 

 crawling upon the stones or plants of the ocean bed. They are termed Nucleobranchiata, 

 because the organs of respiration and digestion are gathered into a mass or nucleus 

 upon the hinder portion of the back. 



The first family of these creatures is represented by the CARINARIA, which may be 

 seen swimming at the upper part of the engraving. In this genus, the gills are protected 

 by a small and very delicate shell of glassy translucence, bearing but little proportion 

 to the size of the animal. The creature itself averages two inches in length, and is very 

 transparent, permitting the vital functions to be watched by the help of a microscope. 

 When swimming, the Carinaria reverses its attitude, and, as seen in the engraving, keeps 

 the tiny shell downwards. Exactly opposite the shell is placed the curiously modified 

 foot which is now no longer useful for creeping, but is formed into a fin wherewith the 

 creature can propel itself through the water, or a rudder by which it can guide its course. 

 The curious mouth is seen in front, and the extremities of the gills are shown just peep- 

 ing from the edge of the shell. 



It is a tolerably quick swimmer, using both tail and fin in its process, and sinking or 

 rising with equal facility. Sometimes the animal is caught without its shell, and the 

 empty shell is occasionally seen floating on the surface. 



ANOTHER species of the same family, the FIROLA, is extremely like the Carinaria, but 

 is without the glassy shell. It attains a considerable size, being sometimes five inches 

 in length. It seems not to be so active as the last-mentioned species. In the second 

 family of this order, namely, the Atlantidae, the animals are furnished with a well-devel- 

 oped shell, sufficiently large to contain them when they desire to contract themselves 

 within its precincts. 



A SMALL, but important, group of molluscs now comes before us. These are the 

 Pteropoda, or Wing-footed Molluscs, so called from the fin-like lobes that project from 

 the sides, and are evidently analogous to the similar organs in some of the sea-snails. 

 These appendages are used almost like wings, the creature flapping its way vigorously 

 through the water, just as a butterfly urges its devious course through the air. They are 

 found in the hotter seas, swimming boldly in vast multitudes amid the wide waters, and 

 one species (Clio borealis) has long been celebrated as furnishing the huge Greenland 

 whale with the greater part of its subsistence. We will now examine in detail our 



