Fig. 177. A, Hyalæa; B, Clio.
The Clio pyramidata (B, [fig. 177]) is an elegant animal belonging to the same class. Its fragile transparent shell has the form of a triangular pyramid; and from its base proceeds a slender spine, and a similar spine extends from each side of the middle of the shell. The posterior part of the body is globular and pellucid, and in the dark it is vividly luminous, shining through the glassy shell. The fins of the Hyalæa and Clio or Cleodora are of a bright yellow, with a deep purple spot near the base. Both are inhabitants of the ocean.
Fig. 178. Clione borealis.
The Clione borealis ([fig. 178]), which exists in millions in the Arctic Seas, is the most remarkable instance of the Naked Pteropods. It has neither shell nor mantle; its membranous body is not more than half an inch long, its head is formed of two round lobes, on each side of the neck there is a large muscular wing or fin; in swimming the animal brings the tips of the fins almost in contact, first on one side of the neck and then on the other. In calm weather, they come to the surface in myriads, and quickly descend again. There is a pair of slender tentacles close to the head, which are organs of feeling, a pair of eyes are placed on the back of the neck, and acoustic vesicles lined with cilia keep otolites in motion. Besides these organs of sense, the Clione has respiratory, digestive, and nervous systems. The latter consists of a nerve-collar round the gullet, with two nerve masses in its upper part, so the Clione is well supplied with nerves.
Upon each of the two round lobes of the head, there are three tentacles, totally different from those of feeling. They are, in fact, organs of prehension, which can be protruded or withdrawn at pleasure into a fold of the skin. When protruded, these six tentacles form a radiating crown round the mouth, which is terminal, and furnished with fleshy lips. Each of these tentacles is perforated by numerous cavities, appearing like red spots to the naked eye; however, Professor Eschricht discovered that each spot consists of a transparent sheath, enclosing a central body composed of a stem terminated by a tuft of about twenty microscopic suckers, capable of being thrust out to seize prey. The whole number of these prehensile suckers in the head of one Clione was estimated by Eschricht to amount to 330,000. Notwithstanding the vast prehensile power and multitude of these animals, they find abundance of food in the Arctic Ocean, for although the water is generally of the purest ultramarine blue, one fourth of the Greenland Sea, extending over 10° of latitude and some hundred feet deep, is green and turbid, with a profusion of minute animal life. The indefinite increase of the Clione borealis is checked by the whales, who feed upon them, and other minute inhabitants of the Arctic Seas. The Pteropods first appear in a fossil state in the Lower Silurian strata.
Naked Cephalopods.
The Naked Cephalopods have an internal skeleton instead of a shell, in the shape of a transparent horny pen in the Calamary, or the well-known internal shell of the Cuttle Fish; they are divided into Octopods and Decapods, according to the number of their tentacles: the Poulpe, or Octopus vulgaris, is a type of the first, the Sepia or Cuttle Fish, [fig. 179], and the Loligo vulgare or Squid, are types of the last. These creatures may be seen on rocky coasts, or in the ocean hundreds of miles distant from land. They are nocturnal, gregarious, carnivorous, and fierce,—their structure enables them to be tyrants of the ocean. They are strange-looking, repulsive creatures, with staring bright-coloured eyes, while crawling awkwardly on their fleshy arms head downmost; yet they are the most highly organized of all mollusks.
