Lab 6: Mollusca - Class Gastropoda
7 October 1999
Phylum Mollusca
(= soft): snails, clams, octopods, etc.Molluscs are one of the largest animal phyla and inhabit marine, freshwater, and terrestrial environments. Molluscs, bilaterally symmetrical protostome coelomates (the coelom is reduced), have three main body regions: head, visceral mass and foot. The circulatory system is open. All molluscs have a mantle (= pallium), which often secretes the shell (composed of CaCO3 and protein). Water circulates through the mantle cavity, which contains the gills (= ctenidia) in aquatic molluscs. Molluscs exhibit cephalization, an advanced nervous system with a dorsal cerebral ganglion. Molluscs have trochophore and veliger larvae. (see Wallace & Taylor pp. 135-163)
Molluscan Classes
(Class Caudofoveata - wormlike, shell-less, cuticle, tentacles, limited info available.)
(Class Aplacophora - small, wormlike, shell-less, many with ventral groove.)
(Class Monoplacophora – single-plate shell, flat, creeping, nephridia. includes Neopilina.)
Class Polyplacophora
(= bearing many plates): chitons.Chitons are an entirely marine group, often found in the rocky intertidal. The shell consists of 8 overlapping, articulating plates embedded in the thick mantle tissue on the dorsal surface of the body. Chitons, like gastropods, have radulae. Chitons lack eyes but have dorsal, photosensitive aesthetes.
Examine a preserved specimen of the chiton Katharina sp. Follow the procedure outlined in W&T (p. 137). Observe external anatomy only.
IDENTIFY: mouth, foot, shell plates (know which is anterior and posterior), ctenidia (= gills), pallial groove (= mantle cavity), mantle (= girdle).
Class Gastropoda
(= stomach foot): snails & slugsThis is the largest (most diverse) class of molluscs, and its members inhabit marine, freshwater, and terrestrial environments. A typical snail consists of a visceral mass encased in a shell atop a muscular foot. It locomotes via a series of pedal waves resulting from muscular contraction of the foot. During development, gastropods undergo a unique phenomenon called torsion, in which the body twists 180o counterclockwise, resulting in a complete rearrangement of the internal and external anatomy. Each gastropods has a radula, a structure used in feeding.
Class Gastropoda, cont’d.
Subclass Prosobranchia: marine snails; shell with operculum. Prosobranchs are mostly marine and comprise the largest subclass of the gastropods. Many biologists believe that the ancestral gastropod was a prosobranch. Most prosobranchs have a shell, mantle cavity, osphradium, ctenidia, radula and a foot. Torsion is complete in prosobranchs and occurs during larval development.
Acmaea testudinalis (limpet)
Busycon
Busycon - Radula
Busycon canaliculatium
Busycon carca
Busycon contrarium
Busycon perversum
Carinaria
Cassis
Crepidula
Crepidula fornicata
Crepidula nana
Haliotes (abalone)
Haliotes tuberculata
Helix
Littorina littorea
Littorina obtusata
Lunatia heroes
Murex
Murex eggs
Oliva
Polinices
Polinices (hermit crab)
Polinices (sand collar)
Strombis alatus
Urosalpinx cinera
Subclass Opisthobranchia: marine snails that have a reduced shell or have lost the shell entirely. Most opisthobranch species are marine. The opisthobranchs can be distinguished from the prosobranchs by the reduction or loss of several structures: the shell, operculum, mantle cavity and ctenidia. Furthermore, torsion is limited and occurs only during embryogensis in the opisthobranchs. Most opisthobranchs have a second pair of tentacles, rhinopores, that are believed to be chemosensory in function.
sea butterfly
Aplysia
Nudibranch
Subclass Pulmonata: mostly terrestrial snails and slugs, some freshwater snails, few marine snails. Most pulmonates are herbivores and mainly inhabit terrestrial and freshwater environments. Some species have a shell, though in others the shell may be reduced or absent. Only a few species have an operculum but most species do have a radula. The mantle cavity in the pulmonates is highly vascularized and functions as a lung. Pulmonates also undergo torsion but the body is only twisted 90°.
Helix
Melampus bidentatus
Examine and Dissect a specimen of the land snail Helix sp. Following the procedure outlined in W&T (pp. 142-144).
identify the following structures (refer also to Brusca fig. 47D, p. 754):
EXTERNAL: head, foot, shell, tentacles, collar, pneumostome, genital pore
INTERNAL: mantle, brain, buccal bulb, esophagus, salivary glands, stomach, intestine, digestive gland, pericardial sac, heart, anus, penis, vagina, flagellum, seminal receptacle, dart sac, mucus glands, sperm duct (= vas deferens), oviduct (assoc. with capsule gland), spermatheca, albumin gland, hermaphroditic duct
Class Scaphopoda (= spade foot): tusk shells
Most scaphopods are deep marine organisms and live in sand or mud. Scaphopods are very old in the fossil record. Members of this class have a foot, mantle, mantle cavity, radula, and shell. Unlike other molluscs, scaphopods do not have ctenidia (gills) or a circulatory system. Specialized tentacles, captaculae, are located anteriorly and function to capture food. Do not mistake a scaphopod for a cephalopod!
Examine the specimen of Dentalium sp. preserved in acrylic on demonstration. Refer to Brusca fig. 9 (p. 708) to help you identify the captacula (you can't see them, but figure out where they should be located), mouth, foot, shell.
Class Bivalvia
- future lab. 2 hinged valves, no head or radula. Most are filter feeders.Class Cephalopoda
- future lab. Arms with adhesive suckers, well-developed eyes, jaws, radula.