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 They are, in fact, organs of prehension, which can be protruded or withdrawn at pleasure into a fold of the skin.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. [Illustration: Fig. 179. Cuttle Fish.] They have a distinct brain, enclosed in a cartilaginous skull, and all their muscles are attached to cartilages. The lower part of their body is surrounded by a mantle, which extends in front to form a gill chamber, in which there is a pair of plume-like gills; a funnel or siphon projects from the gill chamber immediately below the tentacles. All the naked Cephalopods propel themselves back foremost in the sea, by the forcible expulsion of water from the gill chamber through this siphon.