Part 28

Snails and slugs possess a head which bears unjointed tentacles, and also eyes, which, in many cases, are borne on the tips of these. The foot serves for creeping, and is flattened in a sole-like way on its under side. The mantle is generally small, and limited to the dorsal side of the animal. The usually spiral shell covers therefore only a small part of the body, but the rest of the body can also be mostly or entirely drawn back into it. Snails possess jaws, but the chief organ used in attacking the plant structures is the swollen tongue, covered by a rasping plate (radula), on the surface of which regular rows of teeth are arranged. The rest of the tongue consists of muscle, and two small cartilages are also found within it. The teeth stick up when the tongue is protruded, and the whole apparatus forms a kind of rasp, by the backward and forward movements of which the food is reduced to small fragments. Marine snails breathe by gills, which are found in the mantle-cavity, while most of the freshwater and all the land forms (“Pulmonata”) breathe by means of the inner wall of the mantle-cavity, which thus serves as a lung. Some members of the group are hermaphrodite, while the sexes are distinct in others; the first is the case in all terrestrial forms. Such snails pair, however, mutually fertilizing each other. They are not able to fertilize themselves. It is only among land snails and slugs that injurious kinds are found. Species of the genus Helix, e.g. the =Edible= or =Roman Snail= (H. pomatia) and the =Field Snail= (H. nemoralis), may do harm in fruit-culture; =Slugs= are very injurious to agriculture (Limax and Arion); in Limax, the respiratory opening is situated behind the middle of the right margin of the mantle, while the genital opening lies behind the right feeler; in Arion, the respiratory opening is situated on the front edge of the mantle, and the genital opening immediately in front of it. Any kind of snail or slug may be harmful, especially if it attacks young plants. Some kinds, however, live chiefly in woods, feeding upon toadstools and dung, perhaps also on bark and weeds (e.g. dandelion). This is especially true of the larger kinds, but these too, if they multiply very rapidly, may effect much damage in gardens and fields. The commonest slugs are—

The =Common Black Slug= (Arion ater).—About four inches long, and generally black.

The =Grey Field Slug= (Limax agrestis, Fig. 144).—Four-fifths of an inch long, small and smooth, with dark tentacles, grey back, often a reddish sheen, and bright grey or white under side.

The =Large Slug= (Limax maximus).—Up to six inches long, with black flecks on the hump-like mantle, and black streaks on the rest of the body.

In a large majority of cases where slugs appear to an injurious extent, the grey field slug is the offender, and I will therefore deal with it in greater detail. What is said regarding the small species, however, may be taken to apply to all the other kinds, should they become a pest.

The =Grey Field Slug= (Limax agrestis).

FIG. 144.—The Grey Field Slug (Limax agrestis). ]

The life of the grey field slug is very dependent on moisture. During dry weather it rolls itself up and hides in the ground, under a stone, or in some such place, where it is protected as much as possible from completely drying up. At such times it only comes to the surface to feed at night, but does so during the day as well when the weather is wet. The field slug is very destructive in fields and gardens, but this is only the case in those years the great dampness of which is specially favourable to its continuance and multiplication. The glassy eggs are laid in heaps containing from six to fifteen, in damp concealed spots in the ground, under moss, grass, etc., during August, September, and even October. A single slug may lay as many as four hundred. The eggs are able to withstand extreme cold and extreme dryness, and moisture makes them swell out again if they have shrivelled up. In warm, damp weather the young slugs (which are one-twelfth of an inch long in the contracted, and two-fifths of an inch in the extended condition) hatch out in three or four weeks. Cold weather retards the development. If the weather is favourable, and suitable food is not wanting, they are often almost full-grown in six weeks’ time. It is clear, from the foregoing, that in damp years the slugs may appear in very large numbers, especially in autumn; but it may also happen that most of the young slugs hatch out the following spring, in which case the greatest amount of damage will be done then. A snail may live several years, spending the dry days and the whole of the cold season in the ground or some other sheltered place. These slugs devour by preference delicate leaves and stems; young grain, young clover plants, leaves of rape, cabbage, turnips, and lettuce are their favourite food, also fallen fruit, strawberries, cucumbers, and vegetable marrows. Preventive Measures: Protection of their natural enemies: pigs, moles, shrews, ducks, fowls, pigeons, rooks, starlings, toads. Draining the soil, as slugs flourish most where it is moist. Sowing the winter grain as soon as possible, as the young plants will then probably have attained a fair size when the slugs appear in force. Remedies: Collection. The slugs readily crawl on to bits of cucumber or turnip which are thrown down, and can then easily be collected, or killed with quicklime. All substances which readily absorb water kill slugs by taking from them the moisture which is necessary for their existence. Cooking-salt, unslaked and slaked lime may be employed for this purpose. It seems to me most convenient to use freshly slaked lime, ten to twelve bushels per acre. Since the lime can only kill the slugs when it comes into contact with their skins, it must always be strewn on the land at a time when they are on the crawl, not later than 8 a.m., and only in dry weather. As soon as the slugs are touched by the lime they begin to give out a great deal of slime, and many of them, especially the younger ones, are killed and become black. Very many of the older slugs, however, escape death, since the tenacious slime they give out, when the lime touches them, forms a continuous coat like a cocoon, out of which the slug creeps, now much poorer in water. It is therefore a good plan to strew powdered lime over a slug-infested field twice in succession, leaving an interval of some ten or fifteen minutes. The second strewing brings the lime into contact with those slugs which have just crawled out of their slimy investment in a desiccated condition. They are unable to form a second coating of slime, and if touched for this second time with even the smallest particles of lime they at once contract and die. If a double strewing is repeated very early on the following morning, no slugs will be left alive on the field so treated.

CLASS: =LAMELLIBRANCHIATA= (BIVALVE MOLLUSCS).

Headless. The mantle consists of two flaps, which secrete two shelly pieces (valves) movably united on the dorsal side (Fig. 43). The foot is usually wedge-shaped. Respiration effected by gills placed in the mantle-cavity, which communicates with the exterior either by the usual respiratory opening, or by a breathing-tube (siphon). Most species are marine, but some live in fresh water. No kind is of agricultural importance. To this class belong the =Oyster= (Ostrea edulis), the =Sea Mussel= (Mytilus edulis), the tropical =Pearl Mussel= (Meleagrina margaritifera), the freshwater =Swan Mussel= (Anodonta cygnea), the notorious =Ship Worm= (Teredo navalis), etc.

=Fifth Sub-Kingdom=: =ECHINODERMATA= (HEDGEHOG-SKINNED ANIMALS).

FIG. 145.—The Common Starfish (Asterias rubens), seen on the right from above, on the left from below. ]

The echinoderms are radially symmetrical, i.e. a number of similar parts which together make up the body are arranged around an axis (Fig. 145). These animals are usually five-rayed. The mouth lies at lower end of the axis of the body. In some echinoderms the radial symmetry is indistinct, and at the same time there is a more or less clear bilateral symmetry (p. 16). The dermal skeleton either consists of numerous microscopic calcareous deposits, or of firm plates; the skin often bears spines, hence the name of the group. Echinoderms move by means of rows of tubular feet, which are connected with a water-vascular system, and are protruded by means of small bladders, connected with the same system, which contract and force water into them. The form of the body is apple-shaped to spherical (=Regular Sea Urchins=), shield- or heart-shaped (=Irregular Sea Urchins=), pentagonal to star-shaped, with arms of varying length (=Starfishes=), or cylindrical to vermiform (=Sea Cucumbers=). All echinoderms are marine, and none of them, therefore, are of agricultural importance, though near the coast starfishes are sometimes used as manure.

=Sixth Sub-Kingdom=: =CŒLENTERATA= (ZOOPHYTES).

With a few exceptions the zoophytes are radially symmetrical (p. 252), and four-, six-, eight-, or many-rayed. They consist of a body-wall enclosing a cavity which, in the simplest forms (Fig. 146), is sack-like, but in several of the higher sub-groups divided up, and serves, not only for the reception and digestion of food, but also for the circulation of the digested food. Many zoophytes develop hard parts in the walls of their bodies, which remain after their death. The red coral used for ornamental purposes is of this nature, and so also are the coral islands which polypes build up. With the exception of the sponges, which are usually reckoned as belonging to this sub-kingdom, these animals possess “nettling organs” with which they kill their prey, and by which they are protected from attack. They reproduce by eggs, and most of them by means of budding as well (Fig. 146). In many species the buds are not detached as in the Freshwater Polype (Hydra), but remain attached so that a “colony” is formed. There are also free-swimming cœlenterates, known as jellyfishes or medusæ (Fig. 147). Many of these lay eggs which do not develop into fresh medusæ, but into polypes, which increase by budding to form colonies, on which medusæ are once more developed as buds. There is thus an alternation of generations (p. 16), in which the medusæ represent the sexual and the polypes the asexual stage.

FIG. 146.—Freshwater Polype (Hydra), bearing two buds of different ages. Highly magnified. ]

FIG. 147.—A Phosphorescent Jellyfish (Pelagia noctiluca), ½ the natural size. ]

FIG. 148.—A Sea Anemone (Sagartia nivea). ]

Almost all zoophytes live in the sea, but some few in fresh water; none are important agriculturally. The following belong to this sub-kingdom: 1. The =Hydroid Zoophytes= (Hydrozoa), including the medusæ of the North Sea, many of which are the sexual stage in the life history of fixed asexual polypes. The common =Freshwater Polypes= (Hydra viridis, fusca, etc.). 2. The =Flower Polypes= (Anthozoa), to which belong =Red Coral= (Corallium rubrum), and the =reef-building corals= (Astræa, Mæandrina, etc.). 3. The =Sponges= (Porifera), which are usually considered cœlenterates, though they possess no nettling organs (=Freshwater Sponge= = Spongilla, =Bath Sponge= = Euspongia officinalis).

=Seventh Sub-Kingdom: PROTOZOA= (ONE-CELLED ANIMALS).

FIG. 149.—Two species of Proteus Animalcule (Amœba), one with protoplasmic processes, slender and branched; the other with short blunt processes. Highly magnified. ]

This sub-kingdom includes animals which are mostly microscopic, or at any rate small, and live in the sea, fresh water, or damp places. They are unicellular, and consist of a soft viscous substance (protoplasm or sarcode) of contractile nature, and therefore capable of altering its shape. They may or may not be enclosed by a firm wall. Many secrete siliceous structures or calcareous shells. They are of no agricultural importance. Here belong: =Infusoria=, common in putrid solutions; and =Rhizopods=, such as Amœba, the =Proteus Animalcule= (Fig. 149), so called because its shape is constantly altering.

THE END.

PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES.

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OTHER WORKS BY PROF. AINSWORTH DAVIS.

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=A TEXT-BOOK OF BIOLOGY.= Second Edition. Part I., =Botany=, 8s. 6d. Part II., =Zoology=, 10s. 6d.