40 



THE POPULAE EDUCATOE. 



creatures have very elaborate organs for seizing and holding 

 prey, they are so minute as to require high powers of the 

 microscope to detect them at all. The organs of sense are but 

 ill-developed. Thus, though they certainly occupy a position 

 between the Gasteropoda and Cephalopoda, they seem in some 

 respects inferior to both. The mass of internal organs of the 

 animal occupies a much smaller space than the interior of the 

 shell. They are carnivorous, feeding on small animals. The 

 mouth leads, in hyalea, into a narrow throat, and this into a 

 round stomach. The short intestine opens into the cavity 

 between the mantle walls. They are opistho branchiate that 

 is, their gills are situated behind the heart. The heart is, as in 

 all molluscs, systemic, and receives blood from the gills, and 

 propels it to the body. The liver is large, and there is below it 

 an organ supposed to perform the functions of a kidney. 



The nervous system consists of a central mass, composed of 

 two ganglia, united by a band which passes under the throat, and 

 this sends off nerves to the wings and mantle. In Clio Borealis 

 there are four large ganglia and two small ones in the same 

 position. This creature has a bi-lobed hood, which can cover all 

 the head, while the tentacles run through it, and so warn the 

 animal, by the sense of touch, of any external object ; and then, 

 if this object be prey of any kind, it throws back its hood and 

 exposes six organs, placed three on each side of the mouth, 

 which are studded over with an immense number of tubes, each 

 of which can protrude from its end twenty organs which can act 

 ai suckers, and so their minute victims are secured and passed to 

 the triangular mouth, which is furnished with small, singularly- 

 shaped jaws. In Clio Borealis there are two round dark spots at 

 the back of the hood to which nerves run, and these were once 

 supposed to be eyes; but as little stones have been found in 

 these organs, they are now supposed to be ears. The sexes are 

 united in each individual. 



CEPHALOPODA. 



This last and highest class of the Mollusca differs from the 

 rest in containing animals with far higher powers of locomotion 

 and perception than any of the others. The different species 

 are, it is true, often very uncouth and grotesque in appearance, 

 but probably the grotesqueness is due to the fact that they 

 seldom come under our notice. Every creature which we have 

 never observed before, and which differs in external form from 

 those with which we have been previously acquainted, always 

 creates the impression of outlandishness, however well it may be 

 adapted to its own conditions of life. If we were to account 

 utrange and grotesque those forms which differ most from the 

 type upon which most creatures are formed, both man and the 

 horse would be thought very strange creatures. When, however, 

 we find organs whoso uses we know well, and with whose out- 

 ward form the eye is familiarised, blended with other organs 

 which have never before come under our notice, no doubt the 

 impression of uncouthness is strongest. Thus, the fact that a 

 cuttle-fish has large eyes on each side of its head very much like 

 our own, and also a beak like that of a parrot, united with a 

 body like a leathern bag, from the mouth of which stretch long 

 arms studded with sucking cups, makes this creature not only 

 appear singular, but even disgusting to some. 



In the higher examples of Articulata we find that as they 

 become more organised and complicated in structure, and better 

 suited to the accomplishment of the noble vital functions, so 

 do they tend to differ from all other creatures in the other 

 branches of the animal kingdom. We may, perhaps, assume 

 that the branch Vertebrata contains the highest of all animals ; 

 but in proportion as insects become perfected, so far do they 

 differ from vertebrates. Though the functions be the same, the 

 methods by which they are performed differ utterly. The 

 faculties of perception and locomotion are some of the highest 

 animal powers, and these are possessed in quite as large measure 

 by the dragon-fly as by man or the dog ; but the instruments by 

 which the former moves and sees are not only quite different 

 from those employed by the latter, but they are the more 

 different, as manifested throughout the class Insecta, as they 

 become more perfect. On the other hand, as the Mollusca 

 become more highly organised they become more like the 

 Vertebrata, and most of all like them in those organs which 

 minister to the higher functions, for which the sub-kingdom is 

 not noted. Thus, not only does the eye of the cuttle-fish much 

 rosemble that of a vertebrate, but, associated with the greater 



powers of perception and locomotion, is the development of a 

 large concentrated brain, enclosed in some cases in a carti- 

 laginous box, from which prolongations are extended to shield 

 and support the sense-capsules (ears and eyes), and also to sup- 

 port the organs of motion. This cartilage seems to be the true 

 representative or homologue of the internal skeleton of the ver- 

 tebrates, and in this class it becomes developed from the merest 

 rudiment until it entirely supplants the shell, which we find, not 

 only in this class, but in the other classes of the Mollusca, play- 

 ing the part, not merely of a protection, but also a fulcrum, or 

 fixed hard part, from which muscles could move the soft parts 

 of the body. However much we might wish it otherwise, we 

 must, therefore, consider ourselves more nearly allied to the gross, 

 dull, and sluggish Mollusca, than to the active and graceful 

 articulates ; but though the gap in the series which cuts off the 

 vertebrates from the invertebrates is doubtless the most decided 

 and definite which is found in the whole animal kingdom, yet 

 the cephalopods furnish a link which connects us with the 

 Mollusca, while there is no such link between the articulates and 

 the branch to which we belong. 



The Cephalopoda are so called because the integument of the 

 body is drawn out round the mouth into long, tapering, flexible 

 thongs, which are the instruments which have to serve, not only 

 as feelers and arms, but also as legs. In this latter capacity 

 they are used when the animal makes its way over solid ground, 

 and, from their position, the animal must of course walk upon its 

 head, and hence the name head-footed, or head walkers. This 

 mode of progression is, however, seldom resorted to, as the 

 creatures are oceanic rather than terrestrial, and made for 

 swimming rather than walking. They only approach the 

 shore to lay their eggs, but swim the sea in order to procure 

 prey. Not unfrequently, however, they have a retreat in the 

 dark cranny of some submarine rock, in the neighbourhood of 

 which the refuse of their prey is seen to accumulate. 



The cephalopods are divided into two great divisions, called, 

 according to the number of their gills, Tetrabranchiata and 

 Dibranchiata ; or, according to the number of their arms, 

 Tentaculifera and Acetabulifera. To the former belongs the 

 pearly nautilus, whose shells are so commonly seen in naturalists' 

 shops, but which belong to at most only two species of animals. 

 All the rest of this once numerously represented sub-class are 

 fossil. The ammonites of the secondary period all give indica- 

 tions that they belong to this division, and their variety of form 

 and number the number of individuals which lived during that 

 period is truly amazing. It is curious that, while all the am- 

 monites have died out, the nautilus, which still exists, represents 

 a genus which lived in the primary ages, long before the former 

 came into existence. It is probable that no other genus, and 

 certainly none so high in the animal scale, has had so prolonged 

 an existence on our planet. Since the Nautilus pompilius and 

 the Nautilus umbilicatus are the only two existing species, we are 

 compelled to interpret the structure of all the soft organs 

 possessed by this class by an examination of these species only, 

 and even this examination is difficult to accomplish ; for though 

 the shells of these creatures are comparatively numerous, and 

 are found washed up at the sea margin, the animals are oceanic 

 and very wary, so that they can only be captured on rare 

 occasions. In fact, we are dependent upon the investigations of 

 two anatomists only, Mr. Owen and M. Valenciennes, for a 

 description of the nautilus. 



The pearly nautilus has a shell rolled lip in a spire, whose 

 whorls are all in one plane that is, the outer circles are wound 

 evenly round the inner ones, as a piece of flat tape is wound 

 upon itself. This method of rendering the shell compact is very 

 general throughout the class (though not absolutely universal), 

 and serves to distinguish the shells of Cephalopoda from those of 

 the Gasteropoda, which are never wound evenly round the central 

 portion, but always to one side of the plane, in which the pro- 

 ceeding whorls lie just in the same manner as the string of 

 a peg-top is wound around it. This shell is divided into a 

 number of chambers, all of which communicate with each other 

 by a little tubular opening, situated in the centre of each par- 

 tition. Though these chambers and the communicating tubes 

 are lined with live membrane proceeding from the animal, all 

 the essential parts of that animal are contained in its bag-like, 

 short, cylindrical body, which is lodged in the last large chamber 

 of the shell, whose orifice is wide. As the nautilus grows it 

 secretes more shell from its mantle, thus extending the mouth 



