obtains access to the gills by means of a tubular prolongation or folding of the mantle, forming a "siphon," the effete water

Fig. 106. -Diagrammatic section of a Whelk. a Mouth, with masticatory apparatus;

Salivary glands; c Stomach; dd Intestine, surrounded by the liver, and terminat ing in the anus (e); g Gill; h Heart; Nervous ganglion.

being expelled by another posterior siphon similarly constructed. In the air-breathing Gasteropods, the breathing organ is in the form of a pulmonary chamber, formed by an

Fig. 197.-Ampullaria canaliculata, one of the Apple-shells. Operculum;
s Respiratory siphon.

inflection of the mantle, and having a distinct aperture for the admission of air.

The nervous system in the Gasteropoda has its normal composition of three principal pairs of ganglia, the supra-csophageal or cerebral, the infra-oesophageal or pedal, and the parietosplanchnic; but there is a tendency to the aggregation of these in the neighbourhood of the head. The organs of sense are the two eyes, and auditory capsules placed at the bases of the tentacles, the latter being tactile organs.

The sexes are mostly distinct, but in some they are united in the same individual. The young, when first hatched, are always provided with an embryonic shell, which in the adult may become concealed in a fold of the mantle, or may be entirely lost. In the branchiate Gasteropods the embryo (fig. 198, B) is protected by a small nautiloid shell, within which it can entirely retract itself; and it is enabled to swim freely by means of a ciliated, often lobed extension of the cephalic integument, which is termed the "velum," and which is at first merely a circlet of cilia round the head. The velum has often been compared with the wing-like cephalic fins of the Pteropoda, with which, however, it is only doubtfully homologous. Amongst the Pulmonate Gasteropods, those which are strictly terrestrial, undergo no metamorphosis, the velum being absent altogether, whereas those that live in fresh water possess structures which correspond with the velum of the Branchiate forms.

Shell of the Gasteropoda.—The shell of the Gasteropods is composed either of a single piece (univalve), or of a number of plates succeeding one another from before backwards (multivalve). The univalve shell is to be regarded as essentially a cone, the apex of which is more or less oblique. In the simplest form of the shell, the conical shape is retained without any alteration, as is seen in the common Limpet (Patella). In the great majority of cases, however, the cone is considerably elongated, so as to form a tube, which may retain this shape (as in Dentalium), but is usually coiled up into a spiral. The "spiral univalve" (figs. 199, 200) may, in fact, be looked upon as the typical form of the shell in the Gasteropoda. In some cases the coils of the shell-termed technically the "whorls"—are hardly in contact with one another (as in Vermetus). More commonly the whorls are in contact, and are so amalgamated that the inner side of each convolution is formed by the pre-existing whorl. In some cases the whorls of the shell are coiled round a central axis in the same plane, when the shell is said to be "discoidal" (as in the common freshwater shell Planorbis). In most cases, however, the whorls are wound round an axis in an oblique manner, a true spiral

being formed, and the shell becoming "turreted," "trochoid," "turbinated," &c. This last form is the one which may be

Fig. 198.-A, Young of Eolis, a water-breathing Gasteropod, showing the provisional buccal lobes or "velum." B, Adult Pteropod (Limacina Antarctica). (After Woodward.)

looked upon as most characteristic of the Gasteropods, the shell being composed of a number of whorls passing obliquely round a central axis or "columella," having the embryonic shell or "nucleus" at its apex, and having the mouth or "aperture" of the shell placed at the extremity of the last and largest of the whorls, termed the "body-whorl" (fig. 199). The lines or

Fig. 199.-Anterior and posterior views of Cassis cancellata, a spiral Gasteropod. a Spire," placed at the posterior end of the shell; 6" Mouth," placed at the anterior end of the shell; c Inner or columellar lip; d Outer lip; e Notch for the passage of a respiratory siphon.

grooves formed by the junction of the whorls are termed the "sutures," and the whorls above the body-whorl constitute the "spire" of the shell. The axis of the shell (columella) round which the whorls are coiled is usually solid, when the shell is said to be "imperforate;" but it is sometimes hollow, when the shell is said to be "perforated," and the aperture of the axis near the mouth of the shell is called the "umbilicus."

The margin of the "aperture" of the shell is termed the "peristome," or "peritreme," and is composed of an outer and inner lip, of which the former is often expanded or fringed with spines. When these expansions or fringes are periodically formed, the place of the mouth of the shell at different stages of its growth is marked by ridges or rows of spines, which cross the whorls, and are called "varices." In most of the phytophagous Gasteropods (Holostomata) the aperture of the shell (fig. 200) is unbrokenly round or "entire," but in the carnivorous forms (Siphonostomata) it is notched, or produced into a canal (fig. 201). Often there are two of these canals, an anterior and a posterior, but they do not necessarily indicate the nature of the food, as their function is to protect the respiratory siphons. The animal withdraws into its shell by a retractor muscle, which passes into the foot, or is attached to the operculum; its scar or impression being placed, in the spiral univalves, upon the columella.

In the multivalve Gasteropods, the shell is composed of eight transverse imbricated plates, which succeed one another from before backwards, and are embedded in the leathery or fibrous border of the mantle, which may be plain, or may be beset with bristles, spines or scales.

CHAPTER XLVII.

DIVISIONS OF THE GASTEROPODA.

THE Gasteropoda are divided into two primary sections or subclasses, according as the respiratory organs are adapted for breathing air directly or dissolved in water: termed respectively the Pulmonata, Pulmonifera, or Pulmogasteropoda, and the Branchiata, Branchifera, or Branchiogasteropoda.

SUB-CLASS A. BRANCHIFERA or BRANCHIOGASTEROPODA. In this sub-class respiration is aquatic, effected by the thin walls of the mantle-cavity, by external branchial tufts, or by pectinated or plume-like gills, contained in a more or less complete branchial chamber. Flexure of intestine hamal.

This sub-class comprises three orders-viz., the Prosobranchiata, the Opisthobranchiata, and the Nucleobranchiata or Heteropoda.

ORDER I. PROSOBRANCHIATA.-The members of this order are defined as follows: "Abdomen well developed, and pro

tected by a shell, into which the whole animal can usually retire. Mantle forming a vaulted chamber over the back of the head, in which are placed the excretory orifices, and in which the branchiæ are almost always lodged. Branchia pectinated or plume-like, situated (proson) in advance of the heart. Sexes distinct" (Milne-Edwards) (See Woodward's 'Manual.')

The order Prosobranchiata includes all the most characteristic members of the Branchiate Gasteropods, and is divisible into two sections, termed respectively Siphonostomata and Holostomata, according as the aperture of the shell is notched or produced into a canal, or is simply rounded and "entire."

The Siphonostomata, of which the common Whelk (Buccinum undatum, fig. 194) may be taken as an example, are all marine, and are mostly carnivorous in their habits. The following families are comprised in this section:-Strombida (Wingshells), Muricida, Buccinide (Whelks), Conida (Cones), Volutida, and Cypræida (Cowries).

The Holostomata, of which the common Periwinkle (Littorina littorea) is a good example, are either spiral or limpetshaped, in some few instances tubular, or multivalve; the aperture of the shell being in most cases entire (fig. 200). They are mostly plant-eaters, and they may be either marine or inhabitants of fresh water. The following families are included in this section:-Naticida, Pyramidellida, Cerithiada, Melaniada, Turritellida, Littorinida (Periwinkles), Paludinida (River - snails), Neritida, Turbinida (Top-shells), Haliotida (Ear-shells), Fissurellida (Key-hole Limpets), Calyptraida (Bonnet Limpets), Patellida (Limpets), Dentalida (Toothshells), and Chitonida.

The Dentalida are often regarded as a separate order of the Gasteropods (viz., Scaphopoda), or, by Huxley, as referable to the Pteropoda. They constitute a lowly-organised group, distinguished by the absence of distinct gills or heart, the imperfect development of the head, and the slender tubular shell, with an aperture at each end.

The Chitonida and Patellida are often united into a separate order (Cyclobranchiata), characterised by the generally circular disposition of the branchiæ. The former have a multivalve shell, and are stated to have the sexes united.

By many naturalists, the Prosobranchiate order is divided into suborders, in accordance with the structure and form of the "odontophore or "radula."

ORDER II. OPISTHOBRANCHIATA.-This order is defined as follows: "Shell rudimentary, or wanting. Branchia arbores cent or fasciculated, not contained in a special cavity, bu more or less completely exposed on the back and sides, to