THE PTEROPODA 415 



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 colour, and expired in a very few' moments. 



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

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

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

 accounts for the hydrostatic power the animal possesses. The colour 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 inclosed 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 CAEINAEIA, 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 trans- 

 parent, 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 peeping 

 from the edge of the shell. 



It is a tolerably quick swimmer,. using both tail and fin in its progress, 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 Atlantidee, the animals are furnished with a well-developed 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 boreatis) has long been celebrated as furnishing the huge Greenland whale with the 

 greater part of its subsistence. We will now examine in detail our representative species. 



