410 



THE POPULAE EDUCATOR. 



This is more especially the ease in the carnivorous sea-snails, in 

 which it is associated with a long extensible proboscis. In the 

 land and fresh-water gasteropods belonging to the order 

 Pulmonif era, the number in a cross direction is very great, but 

 the lingual ribbon is much shorter. This tooth-bearing ribbon 

 is set on a muscular pad, which can move it backward and 

 forward, so that the little flinty teeth act as a fine file. It is 

 curious that these teeth are composed neither of horn nor shell 

 (CaCO 3 ), but of silica (SiO 2 ) or flint. They are, of course, liable 

 to be worn away ; but the ribbon is formed from behind as fast 

 as it wears away in front ; and in some species, a considerable 

 length of it lies coiled up in a sao or pouch, which stretches 

 away from the mouth, ready to supply the place of the continual 

 wear and tear. A few examples of the pattern of the teeth are 

 given in the engraving, in which only one transverse row of 

 three different species is given. The mouth is very muscular, 

 and has on its front and upper wall a broad horny jaw, which 

 is flat, with a cutting edge directed downward. It is of various 

 shapes, and is often toothed on its lower edge. In some sea- 

 Bnails the mouth-cavity is furnished with a long trunk, which 

 can be unfolded from within, and used to grasp objects while 

 they are played upon by the file-like tongue. Inside these 

 trunks there is sometimes a toothed circle or collar of pointed 

 fangs, which very much strengthen the hold that the creature 

 has on its prey. It is singular that this tooth-bearing tongue 

 is found universally, not only among the gasteropods, but also 

 among all the higher orders of the Mollusca, so that some 

 classifiers have associated these together as the Odontophora, or 

 tooth-bearers. 



We proceed to describe the alimentary canal as it occurs 

 in the arion, or common black slug, noticing such marked dif- 

 ferences as occur in some other orders. A very small throat 

 leads from the roundish buccal cavity, and this gradually dilatea 

 until it ends in a wider stomach. On the sides of the throat 

 are situated two large glands for secreting saliva ; but, though 

 bound to the exterior of the throat by vessels, they discharge 

 their secretion into the back part of the mouth by two ducts, 

 which pass between the nervous collar and the attenuated 

 portion of the throat. The stomach is of various shapes in the 

 slugs ; but in the example before OB the hind part forms a kind 

 of globular bag, and the two ducts from the liver enter just 

 before this rounded portion. The intestine leads from the globe 

 directly forward, so that this globular part occupies an acute 

 angle in the course of the alimentary canal, and then, after being 

 bent backwards and forwards two or three times, runs to a small 

 orifice situated at the neck of the animal on the right side, close 

 to the aperture of the breathing chamber. The stomach and 

 intestine are closely embraced by lobes of the very large liver, 

 which is so bound to them as to be with difficulty unravelled. 

 In the case- of the spirally-coiled and shell-bearing gasteropods, 

 the largest masses of the liver are situated in the small end of 

 the spiral shell. In the aplysia (the sea-hare), and some other 

 of the gasteropoda allied to it, the interior of the stomach is 

 studded with shelly plates and spines, thus converting it into a 

 gizzard. 



The breathing organs of the Gasteropoda are very various, 

 and they have been made use of to divide the class into sub- 

 classes and orders. Thus there are four main sub-classes 

 founded mainly on these organs. In the Opisthobranchiata the 

 gills are branched like a tree, or gathered together in bundles 

 and placed on the hind parts of the body behind the heart, and 

 are either naked or only partially protected by a fold of the 

 mantle or shell. In the Pulmonifera there is a chamber 

 situated over the neck, and covered completely in by a thick 

 fold of the mantle. It only receives air through an aperture, 

 which can be closed by a muscle running round it. The walls, 

 and especially the floor, of this chamber have large blood-vessels 

 in them, and so constitute a kind of lung in which the blood is 

 aerated. In the Prosobranchiata the arrangement as to the 

 chamber is much the same, but in the chamber lie one or two 

 gills, usually of a comb-like or feathery form, and in these, and 

 not in the walls of the cavity, the blood becomes oxygenated. 

 In the carnivorous sea-snails the aperture is converted into a 

 canal or siphon, which is often very long, and which has an 

 anterior canal in the aperture of the shell for its accommodation, 

 thus constituting the difference between the round-mouthed 

 (Holostomata) and channelled (Siphonostomata) shells, as shown 

 in the illustration. 



All the land, and most of the fresh-water snails have lungs, 

 and belong to the sub-class Pulmonifera, while the sea-snails 

 have gills, and belong to the other sub-classes. Thus we see 

 repeated in the Mollusca the two different kinds of breathing 

 organs which are suited to aquatic and aerial life, which, in the 

 vertebrates, are represented by the gills of fish and the lungs of 

 the higher orders. From this we may infer that a gill is the 

 necessary form of a water-breathing apparatus. 



There is yet another sub-class of gasteropods called 

 Nucleobranchiata, or Heteropoda, which have various forms of 

 breathing organs; but these are so different in the whole of 

 their structure from the rest that it becomes a question whethei 

 they should be classed with the gasteropods at all. 



The central organ, which aids the circulation of the blood, is 

 situated in the typical gasteropods in the partition or diaphragm, 

 as it is called, which lies between the breathing chamber and 

 the chamber containing the viscera. It is always at the hind 

 part of this, and receives the blood from the gills, or central 

 vessel of the lungs, into a chamber or auricle. From this it 

 passes through a valve to the more muscular ventricle, and is 

 driven by this into a vessel which almost immediately divides 

 into two, one of which goes forward to the mouth and foot, and 

 the other backward to the liver and all those organs which are 

 situated in the recesses of the shell or hind cavity of the abdo- 

 men. The blood, thus distributed by vessels, is said to escape 

 from them into the general cavity of the body, and from thence 

 enters by wide openings to the veins which convey it to the gills 

 or lungs. In the case of the lung-breathers it enters the 

 diaphragm from behind, and runs in two main vessels along the 

 margins of this organ, and then sends off smaller vessels or 

 sinuses towards the central vessel. , In the Prosobranchiata the 

 sexes are distinct; but in the Pulmonifera and Opisthobran- 

 chiata the sexes are united in one individual, and the organs in 

 the former are of very complicated and peculiar structure. In 

 the neighbourhood of the heart there is an organ which is con- 

 sidered to be a kidney, which eliminates the azotised products 

 caused by the wear and tear of the vital action. This organ 

 seems also, in some species, to have the office of introducing 

 water into the blood-system from without, as it has an opening- 

 on the one side into the breathing-chamber, and on the other 

 into the pericardium or external heart-chamber. 



The front part of the' mantle-fold, which covers in tho 

 breathing-chamber, is thickened into a collar, and this is the 

 instrument for secreting the shell. The shape and foldings of 

 this edge of the mantle give rise, in the process of growth, to all 

 those beautiful shells whose variety of colours and shape must 

 be known to the reader. 



One of the characteristics of the gasteropods is the immense 

 amount of sticky mucus they are constantly exuding, and which 

 makes, in the land-slugs, a serious draught on their nutritive 

 system. This is secreted by glands all over the skin, but also, 

 in some species, by special larger glands on the back of the 

 neck. 



The 'nervous ganglia, though they probably consist of the 

 same elements as in the Conchifera, are gathered together so 

 as to form a ring round the throat, situated at the narrow part 

 just behind the buccal mass. The muscular system is almost 

 wholly confined to the skin, except that a broad muscle arising 

 from the lower part of the body runs to the head, and slips of 

 this muscular sheet also go up the tentacles, so that, when in 

 contraction, the tubular tentacle and eye-stalks are pulled into 

 the body at the same time as the head is withdrawn. In the 

 common snail the eye-tentacles are the longest, and are set 

 highest on the head, while the lower pair is simply tactile. In 

 many sea-snails there is only one pair of tentacles, the ends of 

 which are feelers, while the eyes are set on the sides or bases of 

 these. The eyes, themselves, are not highly organised, being- 

 little more than a nerve expanded in front of a dish of black 

 pigment, and placed behind a transparent cornea. Ear-sacs, 

 with round ear-stones in thein, are found in many Gasteropoda. 



The Gasteropoda are the most typical class of the Mollusca 

 that is, they are the central group, showing fewer points of 

 relation to the other sub-kingdoms than the other classes, and 

 possessing a very large number of species nearly allied to one 

 another, so that there are fewer gaps in the series. They, in 

 fact, occupy a similar position with regard to the Mollusca as 

 the insects do to the Articulata, or the osseous fishes do to tha 

 Vertebrata. 



