wards the rear (opisthen) of the body. Sexes united" (MilneEdwards). (See Woodward's Manual.')

The Opisthobranchiata, or "Sea-slugs," may be divided into two sections, the Tectibranchiata and Nudibranchiata, according as the branchiæ are protected or are uncovered.

The first section, that of the Tectibranchiata, is distinguished by the fact that the animal is usually provided with a shell, both in the larval and adult state, and that the branchiæ are protected by the shell or by the mantle. Under this section

are included the families of the Tornatellida, Bullida (Bubbleshells), Aplysiada (Sea-hares), Pleurobranchida, and Phyllidiada.

In the second section, that of the Nudibranchiata (fig. 202), the animal is destitute of a shell, except in the embryo condition, and the branchia (rarely absent, as in Limapontia and Rhodope) are always placed externally on the back or sides of the body. This section comprises the families Dorida (Sealemons), Tritoniada, Eolida, Phyllirhoida, and Elysiada. Specimens of the Sea-slugs and Sea-lemons may at any time be found creeping about on sea-weeds, or attached to the under surface of stones at low water. The head is furnished with tentacles, which appear to be rather connected with the sense of smell than to be used as tactile organs; and behind

the tentacles are generally two eyes. The nervous system is extremely well developed, and would lead to the belief that

Fig. 202. Nudibranchiata. Doris
Johnstoni, one of the Sea-lemons.

the Nudibranchs are amongst the highest of the Gasteropoda. Locomotion is effected, as in the true Slugs, by creeping about on the flattened foot.

ORDER III. NUCLEOBRANCHIATA or HETEROPODA. This order is defined by the following characteristics: Animal provided

with a shell, or not, free-swimming and pelagic; locomotion effected by a fin-like tail, or by a fan-shaped, vertically-flattened, ventral fin.

The Heteropoda are pelagic in their habits, and are found swimming at the surface of the sea. They are to be regarded as the most highly organised of all the Gasteropoda, at the same time that they are not the most typical members of the class. Some of them can retire completely within their shells, closing them with an operculum; but most have large bodies, and the shell is either small (fig. 203) or entirely wanting.

Fig. 203-Heteropoda. Carinaria cymbium. p Proboscis; Tentacles; & Branchiæ; s Shell; f Foot; d Disc. (After Woodward.)

They swim by means of a flattened ventral fin, or by an elongated tail, and adhere at pleasure to sea-weed by a small sucker situated on the side of the fin. These organs are merely modifications of the foot of the ordinary Gasteropods; the fin-like tail being the "metapodium" (as shown by its occasionally carrying an operculum), the sucker being the "mesopodium," and the ventral fin being a modified "propodium." The "epipodia" are apparently altogether wanting. Respiration is sometimes carried on by distinct branchiæ, but in many cases these are wanting, and the function is performed simply by the walls of the pallial chamber.

The Heteropoda are divided into the two families Firolida and Atlantida, the former characterised by having a small shell covering the circulatory and respiratory organs, or by having no shell at all; whilst in the latter there is a welldeveloped shell, into which the animal can retire, and an operculum is often present.

SUB-CLASS B. PULMONIFERA or PULMOGASTEROPODA. - In this sub-class of the Gasteropoda respiration is aerial, and is carried on by an inflection of the mantle, forming a pulmonary chamber into which air is admitted by an external aperture. The flexure of the intestine is neural, and the sexes are united in the same individual.

The Pulmonifera include the ordinary land-snails, slugs, pond-snails, &c., and are usually provided with a well-developed shell, though this may be rudimentary (as in the slugs), or even wanting. Though formed to breathe air directly, many of the members of this sub-class are capable of inhabiting fresh water. The common Pond-snails are good examples of these last. The condition of the shell varies greatly. Some, such as the common Land-snails, have a well-developed shell, within which the animal can withdraw itself completely. Others, such as the common Slugs (fig. 204) have a rudimen

tary shell, which is completely concealed within the mantle. Others are entirely destitute of a shell. They are divided into two sections as follows:

Section I. Inoperculata.-Animal not provided with an operculum to close the shell. In this section are included the. families Helicida (Land-snails), Limacida (Slugs), Oncidiada, Limnaida (Pond-snails), and Auriculide.

Section II. Operculata.-Shell closed by an operculum. In this section are included the families Cyclostomide and Aciculida. DISTRIBUTION OF THE GASTEROPODA IN SPACE.-As a class the Gasteropoda have a world-wide range, some forms being exclusively marine, others inhabiting fresh waters, while others, again, live upon the land. Amongst the Prosobranchiates, the entire order of the Siphonostomata, and the majority of the

Holostomata, are marine; but, amongst the latter, the Melaniade and Paludinidæ are confined to fresh waters, and the Cerithiada and Neritida include a number of fresh or brackish water forms. The Opisthobranchiates are exclusively marine, mostly littoral in their habits, but occasionally oceanic. The Heteropoda are exclusively marine and pelagic. Lastly, amongst the Pulmonates many forms (such as the Snails and Slugs) are strictly terrestrial, whilst others (Limnæa, Planorbis, Ancylus, &c.) are found in fresh or brackish waters.

DISTRIBUTION OF THE GASTEROPODA IN TIME.- The Gasteropoda are represented in past time from the Lower Silurian rocks up to the present day. Of the Branchifera the Holostomata are more abundant in the Paleozoic period, the Siphonostomata abounding more in the Secondary and Tertiary rocks, but not attaining their maximum till the present day. The place of the carnivorous Siphonostomata in the Paleozoic seas appears to have been filled by the Tetrabranchiate Cephalopods. The branchiate Gasteropods of fresh water are chiefly represented as fossils by the genera Melania, Paludina, Valvata, and Ampullaria.

The Heteropoda are likewise of very ancient origin, having commenced their existence in the lowest Silurian deposits. The genera Bellerophon, Porcellia, Cyrtolites, and Maclurea, are almost exclusively Palæozoic; Bellerophina is found in the Gault (Secondary), and Carinaria has been detected in the Tertiaries.

The Pulmonate Gasteropoda, as was to be anticipated, are not found abundantly as fossils, occurring chiefly in lacustrine and estuarine deposits, in which the genera Limnæa, Physa, Ancylus, &c., are amongst those most commonly represented. These, however, are entirely Mesozoic and Kainozoic. In the Paleozoic period the sole known representatives of the Pulmonifera are the Pupa vetusta, Pupa vermilionensis, Dawsonella Mecki, and Zonites priscus of the Carboniferous rocks.

CHAPTER XLVIII.

PTEROPODA.

CLASS III. PTEROPODA.-The Pteropoda are defined by being free and pelagic, swimming by means of two wing-like appendages (epipodia), developed from each side of the anterior extremity of the body. The flexure of the intestine is neural.

As to the position of the Pteropoda in the Molluscan scale, they must be looked upon as inferior in organisation to any of the Gasteropoda, of which class they are often regarded as the lowest division. They permanently represent, from certain aspects, the transient larval stage of the sea-snails.

The Pteropods are all of small size, and are found swimming at the surface of the open ocean, often in enormous numbers. Locomotion is effected by two wing-like fins, developed from the sides of the head, and composed of the greatly-developed "epipodia." The true "foot" is rudimentary and rarely dis

Fig. 205.-Pteropoda. a Cleodora pyramidata;
Cuvieria columnella. (After Woodward.)

Fig. 206.-Hyalea tridentata, showing the shell and the lateral fins attached to the sides of the head (f).

tinct, but the "metapodium" is sometimes provided with an operculum (Limacinide). There is usually a symmetrical, glassy, sometimes chitinous, shell (fig. 205), either consisting of a dorsal and ventral plate united, or forming a spiral (fig. 198, B), but in some cases the body is naked, the mantle being absent or rudimentary. The head is rudimentary, and bears the mouth, which is occasionally tentaculate, and which is furnished with an odontophore. There is a muscular stomach and a well-developed liver; and the flexure of the intestine is neural, so that the anus is situated on the lateral or ventral surface of the body.

The heart consists of an auricle and ventricle. The respiratory organ is very rudimentary, and consists of a ciliated surface, which is either entirely unprotected, or may be contained in a branchial chamber.

The ganglia of the nervous system "are concentrated into a mass below the oesophagus" (Woodward), united by a commissure above the gullet; and the eyes are rudimentary.

The sexes are united in all the Pteropods, and the young pass through a metamorphosis, having at first a bilobed ciliated veil attached to the sides of the head.

The Pteropoda are divided into two orders, termed Thecosomata and Gymnosomata; the former characterised by possess