Page:EB1911 - Volume 11.djvu/535

EUTHYNEURA] :Fam. 4.—Buccinidae. Foot large and broad; eyes at base of tentacles; operculum horny. Buccinum. Chrysodomus. Liomesus. Cominella. Tritonidea. Pisania. Euthria. Phos. Dipsacus.


 * Fam. 5.—Nassidae. Foot broad, with two slender posterior appendages; operculum unguiculate. Nassa, marine, British. Canidia, fluviatile. Bullia.


 * Fam. 6.—Muricidae. Shell with moderately long spire and canal, ornamented with ribs, often spiny; foot truncated anteriorly. Murex, British. Trophon, British. Typhis. Urosalpinx. Lachesis.


 * Fam. 7.—Purpuridae. Shell thick, with short spire, last whorl large and canal short; aperture wide; operculum horny. Purpura, British. Rapana. Monoceros. Sistrum. Concholepas.


 * Fam. 8.—Haliidae. Shell ventricose, thin and smooth, with wide aperture; foot large and thick, without operculum. Halia.


 * Fam. 9.—Cancellariidae. Shell ovoid, with short spire and folded columella; foot small, no operculum; siphon short. Cancellaria.


 * Fam. 10.—Columbellidae. Spire of shell prominent, aperture narrow, canal very short, columella crenelated; foot large. Columbella.


 * Fam. 11.—Coralliophilidae. Shell irregular; radula absent; foot and siphon short; sedentary animals, living in corals. Coralliophila. Rhizochilus. Leptoconchus. Magilus. Rapa.


 * Fam. 12.—Volutidae. Head much flattened and wide, with eyes on sides; foot broad; siphon with internal appendages. Valuta. Guivillea. Cymba.


 * Fam. 13.—Olividae. Foot with anterior transverse groove; a posterior pallial tentacle; generally burrowing. Olivia. Olivella. Ancillaria. Agaronia.


 * Fam. 14.—Marginellidae. Foot very large; mantle reflected over shell. Marginella. Pseudomarginella.


 * Fam. 15.—Harpidae. Foot very large; without operculum; shell with short spire and longitudinal ribs; siphon long. Harpa.

Tribe 2.— No jaws. No median tooth in radula. Formula: 1:0:1. Poison-gland present whose duct traverses the nerve-collar.


 * Fam. 1.—Pleurotomatidae. Shell fusiform, with elongated spire; margin of shell and mantle notched. Pleurotoma. Clavatula. Mangilia. Bela. Pusionella. Pontiothauma.


 * Fam. 2.—Terebridae. Shell turriculated, with numerous whorls; aperture and operculum oval; eyes at summits of tentacles; siphon long. Terebra.


 * Fam. 3.—Conidae. Shell conical, with very short spire, and narrow aperture with parallel borders; operculum unguiform. Conus.

The most important general character of the Euthyneura is the absence of torsion in the visceral commissure, and the more posterior position of the anus and pallial organs. Comparative anatomy and embryology prove that this condition is due, not as formerly supposed to a difference in the relations of the visceral commissure which prevented it from being included in the torsion of the visceral hump, but to an actual detorsion which has taken place in evolution and is repeated to a great extent in individual development. In several of the more primitive forms the same torsion occurs as in Streptoneura, viz. in Actaeon and Limacina among Opisthobranchia, and Chilina among Pulmonata. Actaeon is proso-branchiate, the visceral commissure is twisted in Actaeon and Chilina, and even slightly still in Bulla and Scaphander; in Actaeon and Limacina the osphradium is to the left, innervated by the supra-intestinal ganglion. But in the other members of the sub-class the detorsion of the visceral mass has carried back the anus and circumanal complex from the anterior dorsal region to the right side, as in Bulla and Aplysia, or even to the posterior end of the body, as in Philine, Oncidium, Doris, &c. Different degrees of the same process of detorsion are, as we have seen, exhibited by the Heteropoda among the Streptoneura, and both in them and in the Euthyneura the detorsion is associated with degeneration of the shell. Where the modification is carried to its extreme degree, not only the shell but the pallial cavity, ctenidium and visceral hump disappear, and the body acquires a simple elongated form and a secondary external symmetry, as in Pterotrachaea and in Doris, Eolis, and other Nudibranchia. These facts afford strong support to the hypothesis that the weight of the shell is the original cause of the torsion of the dorsal visceral mass in Gastropods. But this hypothesis leaves the elevation of the visceral mass and the exogastric coiling of the shell in the ancestral form unexplained. In those Euthyneura in which the shell is entirely absent in the adult, it is, except in the three genera Cenia, Runcina and Vaginula, developed in the larva and then falls off. In other cases (Tectibranchs) the reduced shell is enclosed by upgrowths of the edge of the mantle and becomes internal, as in many Cephalopods. A few Euthyneura in which the shell is not much reduced retain an operculum in the adult state, e.g. Actaeon, Limacina, and the marine Pulmonate, Amphibola. The detorted visceral commissure shows a tendency to the concentration of all its elements round the oesophagus, so that except in the Bullomorpha and in Aplysia the whole nervous system is aggregated in the cephalic region, either dorsally or ventrally. The radula has a number of uniform teeth on each side of the median tooth in each transverse row. The head in most cases bears two pairs of tentacles. All the Euthyneura are hermaphrodite.

In the most primitive condition the genital duct is single throughout its length and has a single external aperture; it is therefore said to be monaulic. The hermaphrodite aperture is on the right side near the opening of the pallial cavity, and a ciliated groove conducts the spermatozoa to the penis, which is situated more anteriorly. This is the condition in the Bullomorpha, the Aplysiomorpha, and in one Pulmonate, Pythia. In some cases while the original aperture remains undivided, the seminal groove is closed and so converted into a canal. This is the modification found in Cavolinia longirostris among the Bullomorpha, and in all the Auriculidae except Pythia. A further degree of modification occurs when the male duct takes its origin from the hermaphrodite duct above the external opening, so that there are two distinct apertures, one male and one female, the latter being the original opening. The genital duct is now said to be diaulic, as in Valvata, Oncidiopsis, Actaeon, and Lobiger among the Bullomorpha, in the Pleurobranchidae, in the Nudibranchia, except the Doridomorpha and most of the Elysiomorpha, and in the Pulmonata. Originally in this condition the female aperture is at some distance from the male, as in the Basommatophora and in other cases; but in some forms the female aperture itself has shifted and come to be contiguous with the male opening and penis as in the Stylommatophora. In all these cases the female duct bears a bursa copulatrix or receptaculum seminis. In some forms this receptacle acquires a separate external opening remaining connected with the oviduct internally. There are thus two female openings, one for copulation, the other for oviposition, as well as a male opening. The genital duct is now trifurcated or triaulic, a condition which is confined to certain Nudibranchs, viz. the Doridomorpha and most of the Elysiomorpha.

The Pteropoda, formerly regarded as a distinct class of the Mollusca, were interpreted by E. R. Lankester as a branch of the Cephalopoda, chiefly on account of the protrusible sucker-bearing processes at the anterior end of Pneumonoderma. These he considered to be homologous with the arms of Cephalopods. He fully recognized, however, the similarity of Pteropods to Gastropods in their general asymmetry and in the torsion of the visceral mass in Limacinidae. It is now understood that they are Euthyneurous Gastropods adapted to natatory locomotion and pelagic life. The sucker-bearing processes of Pneumonoderma are outgrowths of the proboscis. The fins of Pteropods are now interpreted as the expanded lateral margins of the foot, termed parapodia, not homologous with the siphon of Cephalopods which is formed from epipodia. The Thecosomatous Pteropoda are allied to Bulla, the Gymnosomatous forms to Aplysia. The Euthyneura comprises two orders, Opisthobranchia and Pulmonata.