THE POPULAB "EDUCATOB. 



the nautilus. They have been called Acetabulifera, because of 

 certain curious appendages to their arms. Their arms are not 

 short, numerous, and retractile within sheaths, as those of the 

 nautilus are, but of definite number (eight or ten), long, and on 

 their inner sides that is, towards the mouth they are studded 

 with a single or double row of acetabula. The word acetabulum 

 is the ancient name given to a vinegar-cruet, and was first 

 applied anatomically to the hollow in the pelvis into which the 

 head of the thigh-bone of man is received. Since then it has 

 always been applied to any cup-like organ which has a deeper 

 depression in the centre of the cup. These cups are sometimes 

 stalked, and sometimes set directly to the arms. The cup is of 

 a cartilaginous consistence, while in the pit there is a leathery 

 piston, which can be retracted by proper muscles after the 

 round edge of the cup has been applied to any object. When 

 the piston is retracted it of course creates a vacuum in the cup, 

 and as there are many hundreds of these cups on the arms 

 of the animal, it can lay a very firm hold on its prey, and drag 

 it with great force towards its beak-like mouth. Besides the 

 cup-like discs, or suckers, the arms of some of the cephalopods 

 are furnished with horny hooks, which assist in retaining the 

 prey. Unlike the nautilus, the jaws of the cetopus are horny 

 and sharp, like the bill of a parrot, only the lower jaw protrudes 

 beyond the upper one. 



The shell of these creatures is far less developed than in the 

 other division of this class. For the most part, it is not apparent 

 at all on the outside, but lies loose in the muscular skin of the 

 back, which it supports, and thus provides that the animal can 

 be thrust alone 1 by the working of the siphon or by the fins. In 

 the paper nautilus, however (Argonauta argo), there is a beautiful 

 external shell, which is not divided into chambers. This shell, 

 though it lodges the bag-like body, has no sort of likeness in 

 form to the shape of the body. Thus, when it was naturally 

 supposed that the shell was secreted, as in the case of the 

 other Mollusca, by the mantle, or investment of the body, 

 naturalists could not imagine how so beautiful a structure was 

 tnoulded on so uncouth a form. It has since been ascertained 

 that the shell is formed by two of the arms, which have a large 

 membranous expansion fitted to secrete such a shell. Before 

 the office of these arms was known, they gave rise in their turn 

 to false conjectures, for it was then thought that this creature 

 sat in its shell as in a boat, and, lifting the webbed arms above 

 the surface of the ocean, was driven along as a ship by its sails. 

 In the other octopods there is no representative of a shell, either 

 external or internal. In some of the decapods it is horny, and 

 in the shape of a pen. In others it is calcareous and thick, but 

 nevertheless it is very light, being quite porous, and made up of 

 very thin plates supported by little pillars. This is the case in 

 the pounce-bone of the sepia. In others, again, as in spirula, 

 the shell, though internal, is chambered, and is supposed, as in 

 the case of the chambered shell of the nautilus, to contain air, 

 which lightens the body and can be compressed when the animal 

 wishes to sink. A number of curious fossils, called belemnites, 

 on account of their resemblance to the head of a dart, have been 

 identified as the prolongations of these internal chambered 

 shells, and as these have been sometimes found associated with 

 the hooks and ink-bag found in the cuttle-fish, there can be little 

 doubt about the identification being genuine. The relation of 

 the shell to the extinct animal is shown in the engraving. The 

 funnel or siphon of the Dibranchiata is entire, and not split 

 along its under side. It leads into a gill-chamber, into which 

 also the ova and faeces are discharged. The buccal mass is 

 globular and large. The tongue bears teeth directed backward, 

 but is in part naked, and seems like an organ of taste. The 

 narrow throat leads into a globular stomach or crop. Into the 

 stomach a large blind sack enters, and the intestine is short, as 

 is usually the case with carnivorous animals, as these are. The 

 circulatory system is peculiar in being almost a closed circuit, 

 and in having, not only a heart distributing the blood to the 

 system, after being aerated at the gills, but also in having two 

 distinct hearts driving the blood to the gills. The blood is con- 

 veyed to these branchial hearts through two large venous canals, 

 whose walls have a spongy texture. These large veins are sup- 

 posed to excrete from the blood, by means of the spongy walls, 

 the ammoniacal liquid equivalent to the urine, and since the 

 vessels lie in the water introduced through the funnel and gill- 

 chambers, of course this could become a means of getting rid of 

 matters no longer useful from the nutritive stream. The ink-bag 



is a peculiar organ situated in the recesses of the body, the duct 

 from which is conveyed up and opens behind the funnel. The 

 secretion is under the control of the animal, and when formed it 

 thickens and obscures the water by a copious discharge. It is 

 said that the Chinese made ink from this secretion, and it is 

 still used as a pigment. 



The whole of the Cephalopoda, both recent and extinct, may 

 be classified thus : 



TETRABRANCHIATA. Eyes stalked, jaws shelly, and body 

 attached to the shell by a muscle; gills, four. 



Family 1. Ammonitidce: Shell of many chambers ; that con- 

 taining the body elongated, the aperture guarded by processes, 

 and closed by an operculum ; sutures angulated, and siphuncle* 

 external (or dorsal). 



Family 2. Orthoceratidce : Shell with a small narrow 

 aperture ; siphuncle complicated. 



Family 3. Nautilidce : Sutures simple ; siphuncle central. 



DIBRANCHIATA. Eyes sessile; jaws horny; two gills; an 

 ink-bag. 



Tribe I. OCTOPODA : Eight arms ; eyes fixed. 

 Family 1. Argonautidce : Dorsal arms webbed ; female 



with a shell. 



Family 2. Octopodidcn : Arms webbed between the roots. 



Tribe II. DECAPODA : Eyes movable ; body finned. 

 Family 3. Teuthidce : Fins nearly at the end of body. 

 Family 4. Belemnitidce : Shell represented by a pen, 



terminating in a chambered cone ; siphuncle on the ventral 



side. 



Family 5. Sepiadce : Calcareous gladius ; elongated ten- 

 tacles. 



Family 6. Spirulida : Discordal, pearly, many- chambered 



shell, with ventral siphuncle. 



LESSONS IN CHEMISTRY. XXVI. 



METALS PROPER continued, 

 GOLD. 



SYMBOL, Au COMBINING WEIGHT, 196 '5 SPECIFIC GRAVITT, 19'3. 

 THIS, the most valued of metals, is always found in a native 

 state, frequently alloyed with small quantities of silver, copper, 

 and some of the rarer metals. Generally its matrix is quartz 

 rock, but by aqueous action this rock becomes degraded into 

 sand, and the gold is carried down to the river-bed. From this 

 source it was chiefly obtained before the discovery of the 

 Calif ornian and Australian gold-fields. It is separated from the 

 Band by " washing " that is, the sand, largely mixed with 

 water, is allowed to run from one " cradle " to another, the light 

 sand being thus washed away, and the heavy particles of gold 

 remaining. In California and Australia the metal is found in 

 lumps " nuggets " which have every appearance of having 

 been fused ; or it is extricated from the quartz. The rock is 

 crushed; and the gold separated by " washing." The process of 

 amalgamation has also been resorted to. The crushed quartz is 

 mixed with mercury, which takes up the gold. The metals are 

 afterwards separated by distilling off the mercury. 



Gold is one of the heaviest of the metals, being only inferior 

 to iridium and platinum. It is very malleable, being capable of 

 being beaten out so thin as to allow green light to pass 

 through it. 



It is very soft. Bracelets of pure gold can be twisted round 

 the arm. It melts at 1100 Cent. 



No simple acid, except selenic, will dissolve it ; but it is acted 

 on by any mixture which liberates chlorine. Such a mixture 

 is " aqua regia," which is composed of 1 of nitric and 4 of 

 hydrochloric acid. The result of this action is auric tri-chlorido 

 (AuCl 3 ). 



From this salt pure gold may be obtained by precipitating 

 the metal from its solution by ferrous sulphate, according to 

 this equation : 



3FeSO 4 + AuCl 3 = Fed, + Fe,3SO + Au. 



The gold falls as a brown powder, which is purple by trans- 

 mitted light. Oxalic acid and the chloride of antimony have a 

 like effect to ferrous sulphate. 



Gilding is effected by causing gold-leaf to adhere to the aur- 



* By siphuncle is meant tbe tube connecting the chambers of the 

 shell. 



