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 Dendronotus 
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 alge, 
with its numerous branching frond-like respiratory organs floating gracefully in the water. 
Eolis and Doris are also represented by pleasing species. Alysia 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 be 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 
Motuusks, from the fin-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 flapping 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 CARrINARIA. 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 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 delieate glass-like shells found on the broad ocean, called 
Carinarias, Atlantas, Pterotracheas, etc., are embraced. The curious eel-like Leptocephalus 
and the Sagittas are seen in our waters. 
THE 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 CLEopoRA. It is a very beautiful and interesting animal, of which Mr. F. D. Bennett 
writes as follows: ‘‘On 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 alamp. 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 SpPrKE-SRELL. 
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. 
