342 THE SPIKE-SHELL. 



are external, and derive the oxygen from the surrounding medium. Most beautiful forms are 

 seen in this group. A few forms are found on our New England coast. The Dend/ronotus 

 arborescens is an elegant example — the one seen in the above cut is identical with ours. Pro- 

 tective resemblances are common in these forms; the latter appears like a bunch of pretty algae, 

 with its numerous branching frond dike respiratory organs floating gracefully in the water. 

 Eolis and Doris are also represented by pleasing species. Elysia is much like the common 

 slugs or naked snail, but somewhat more decorated. 



NUCLEOBRANCHIATA AND PTEROPODA. 



We now arrive at a new order of mollusks, 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. 



It will be seen that a division is here made of more importance than the ordinary one 

 of Orders, because of striking differences that can only lie thus expressed: The Pteropods 

 are all rather unfamiliar, being pelagic in habits, their delicate forms being borne upon 

 the surface of the great deep. They derive their name, Pteropoda, or Wing-footed 

 Mollusks, from the tin-like lobes that project from the sides, and are evidently analogous 

 to the similar organs in some of the sea-snails. A fine specimen of this group of mollusks 

 is seen in the illustration on page 343. The appendages are used almost like wings, the 

 creature Happing its way vigorously through the water, just as a butterfly urges its devious 

 course through the air. They are mostly found in the hotter seas, swimming boldly in vast 

 multitudes amid the wide waters. 



The first family of these creatures is represented by the Carinaria. In this genus, the 

 gills are protected by a small and very delicate shell of glassy transl licence, 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 keeps the tiny shell downwards. 

 The curiously modified foot of the animal is formed into a fin wherewith the creature can 

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



In the Order Heteropoda the delicate glass-like shells found on the broad ocean, called 

 Carinarias, Atlanta*, PterotracJieas, etc., are embraced. The curious eel-like LeptocepTialus 

 and the Sagittas are seen in our waters. 



Tjie curious figure Hyalea is remarkable not only for the two wide fins which are found 

 in all the family to which it belongs, but for the long appendages which pass through certain 

 apertures in the shell, and trail behind as the creature proceeds on its course. The wings are 

 united by a nearly semicircular lobe. The empty shell is placed below in order to show its 

 curious structure. 



A creature, smaller than the Hyalea, and, with an odd-looking three-pointed shell, is 

 the Oleodora. It is a very beautiful and interesting animal, of which Mr. F. P. Bennett 

 writes as follows : " < >n that part of the body which is lodged in the apex of the shell, there is 

 a small, globular, pellucid body, resembling a vesicle, and which at night emits a luminous 

 gleam, sufficiently vivid to be visible even when it is opposed to the strong light of a lamp. It 

 is the only example of a luminous shell-fish I have ever met with ; nor would the luminosity 

 of this species be of any avail, did not the shell possess a structure so vitreous and transparent. 

 Examples were chiefly captured at night or in the evening. 



An example of an allied genus, notable for the straight-pointed shell, is the Spike-shell. 

 The fins of this little animal are rather narrow, and the apex of the shell soon loses its sharp- 

 ness, being by degrees divided into compartments and gradually broken off. The Spike-shell 

 is mostly found near floating sea-weed. 



