Page 13 - Land Snails of New Mexico

thatareendemicto,i.e.,restrictedto,certainmountainranges, oftentoasinglerangeorpeak.Mostofthesearespeciesthat g r o w to relatively large sizes. F o r e x a m p l e , a l m o s t all o f the 3 0 odd species and subspecies of the genus Ashmunella, discussed below, are endemic to a specific range. They mainly inhabit accumulations of talus, usually in the higher parts of the mountains where they occur. Thus, their numbers are in direct proportion to the extent of the areas of talus available to them. Thispatternofendemism inAshmunella and othergenera, such as Oreohelix and Sonorella, is responsible for the relatively large number of species of land snails inhabiting the state.
located in a mouth region, from which food progresses posteriorlyandisprocessedinadigestivetract.A digestive gland is involved variously in secretion of enzymes and in absorption and storage of nutrients. In ancestors of the gastropods, the anus emptied into the posterior mantle cavity, as noted above. A two-chambered heart was present. A nervous system boasted a prominent collar around the anterior part of the digestive tract, from which arose a pair of nerves that extended backintothefootand anerve(visceral)loop,which innervated the visceral mass.
Itislikely that these endemic "snails on mountaintops" are
analogous to species that are endemic to oceanic islands. F r o m
studies of island biogeography, it is known to be part of an
expected cycle for such insular species to go to extinction. This
may apply to our endemic land snails as well. For example,
fossilevidences show thatinthe Sacramento Mountains, several
kindsofAshmunellasandOreohelixeshavebecomeextinct somemonoplacophoran-likeancestorofthegastropodsthata
during the Quaternary. Opposed to a tendency towards extinction, however, is a tendency for species of land snails inhabiting rugged mountains with deep canyons and high ridges to speciate and produce diverse races, which may evolve into new species.
phenomenon called torsion took place, in which the upper body
THEPULMONATELANDSNAIL General
and terminating anteriorly. Another result of torsion was the
The molluscs are a venerable group, already having shells thatbecame part of the fossil record by the Cambrian Period, in the early Paleozoic Era over 500 million years ago. According to Solem (1979), one order of prosobranch and two orders of pulmonate land snails appear in the fossil record in the late Paleozoic (Pennsylvanian and Permian). Thus, they were among theearlycolonizersoftheterrestrialenvironment,andmore or lesscontemporariesofearlyterrestrialvertebrates.Despitetheir long history, land snails have remained rather conservative in regardtosome characteristicsthatlikelytraceback totheirearly Paleozoic ancestors. Such features include a ventral, muscular, mucus-producing foot upon which the animal crawls. The foot issurmounted by organs of the viscera (visceral mass or hump) and these, in turn, by a fleshy mantle. The mantle might be likened to the roof of a thatched hut that hangs down to form projecting eaves. The space between the eaves and upper walls ofthehut would be comparable to a space under the peripheral margins of the mantle, which is termed a mantle cavity. In the
rearrangement of body parts has been discussed at length, but no
early, aquatic, pregastropod molluscs, it is supposed that the
The aperture is the opening in a snail's shell through which it m a y e m e r g e , a n t e r i o r e n d first. T h e p o s t e r i o r ( o p p o s i t e e n d ) alsoemergesifthesnailismoving about.Inlocomotion,ituses a muscular footlocated on the ventral (bottom) side, and moves a l o n g i n a p a t h w a y o f its o w n s e c r e t e d m u c u s , c o m m o n l y c a l l e d "slime." In early morning, the shiny remains of slime trails often arenoted in areas where snailshave been moving about during the preceding night. A b o v e the foot, dorsally, hidden within the shell, are various internal organs of the visceral mass, which is surmounted by a mantle.
mantlecavitywas bestdevelopedintheposteriorregionwhere
gillswere located, and intowhich the anus and excretory ducts
discharged wastes thatthen passed through the mantle cavity and intothewater.Themantlesecretedashellofcalciumcarbonate
and of an organic material, conchiolin. Digestive-tractfeatures,probablyinheritedby thepulmonate
snails from their remote ancestors, include a feeding device
called a radula. This structure is rather like an elongate tongue
supportedbyacartilaginousbaseandbearing,inmost fromtheheadanteriorly.AllbutonespeciesofNewMexico pulmonates, numerous minute chitinous teeth. Pulmonates are
spared dental bills because old, damaged teeth are lost,
anteriorly,andreplacedfrom behindby new teeth.The radulais
A strangethinghappenedintheevolutionaryhistoryofthe gastropodline.Infact,ifithadn'thappened,we wouldn'thave gastropods at all, but would, instead, probably consider them to be monoplacophorans, limpet-shaped mollusks in which the head is anterior and the main mantle cavity posterior, as described above. Monoplacophorans have a long fossil history, andpresentlyarefoundinsome deep-seaareas.Itseems thatin
parts, including the mantle and posterior mantle cavity, rotated
180° with respect to the more ventral parts of the body. This
placed the gills and openings for excretory and digestive tract
discharge in an anterior position, above and behind the head.
Torsion also caused the digestive tract to form a loop, beginning
twisting of the visceral nerve loop into a figure-eight
configuration. What the advantages may have been of this
explanation has achieved consensus among those studying the
phenomenon. There is agreement that the new position of the
apertures for fecal and excretory wastes was not in the best interests of sanitation. Various solutions have evolved to
counteract this problem. S o m e of these solutions involve the control of water currents in relation to head, gills, and waste discharge. In our pulmonate land snails the problem has been avoided by modifying the mantle cavity into an air-breathing device,more likeourown lungs.From thislung-likestructure has been derived the name Pulmonata (cf. Spanish pulmones, meaning lungs). The lung-like mantle cavity has a breathing pore called a pneumostome, which opens to the outside.
External Anatomy
Sensory tentacles, which bear eyes at their anterior tip, arise
land snailshave two pairs of tentacles--the sole exception being atinysnail,rareinthestate,Carychiumexiguum.NewMexico's pulmonateaquaticsnails,ontheotherhand,haveonlyonepair
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