Chapter 37: Mollusks and Annelids. 37-1 Phylum Mollusca. 37-2 Phylum Annelida. 37-1 Phylum Mollusca. I. Characteristics of Mollusks (“SOFT-body”~ 112,000 species). COELOMATES, aquatic AND terrestrial, and 3 CLASSES of mollusks (some shelled, some unshelled).
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Chapter 37: Mollusks and Annelids 37-1 Phylum Mollusca 37-2 Phylum Annelida
37-1 Phylum Mollusca I. Characteristics of Mollusks (“SOFT-body”~ 112,000 species) • COELOMATES, aquatic AND terrestrial, and 3 CLASSES of mollusks (some shelled, some unshelled).
(1) Trochophore (aquatic larval SHARED among mollusks AND annelids) • Zygote ciliated larvae, propels through water, FREE-LIVING.
II. Body Plan of Mollusks • Divided into “head-foot” AND “visceral mass;” covered by MANTLE, (SECRETES shell).
(1) Head-Foot • Contains MOUTH, GANGLIA (head), and LARGE FOOT (locomotion).
(2) Visceral Mass (contains INTERNAL organs) • Contains heart AND organs for digestion, excretion, and reproduction. NOTE: The COELOM is limited to a SPACE around the heart.
(3) Mantle & Mantle Cavity (outer-covering OVER visceral mass) • Tissue produces a CaCO3 shell; CAVITY is between mantle AND visceral mass.
(4) Ganglia (connected by TWO pairs of long nerve cords) • Control locomotion, feeding, and process sensory information (light, touch, chemicals).
(5) Radula (FEEDING adaptation, diversified) • In MOST mollusks, a tongue-like strip of ABRASIVE tissue with teeth for scraping, piercing, or cracking.
III. Class Gastropoda (“STOMACH-foot”, mobile, ~ 90,000 species) • Snails, abalones, conchs, slugs, nudibranchs; most have a SINGLE shell, (some have lost their shells through evolution).
(1) Torsion (DEVELOPMENTAL process of gastropod LARVAE) • Visceral mass TWISTS around the HEAD region, resulting in a mantle cavity, gills, and anus (makes a space to WITHDRAW head if threatened).
(2) Hemolymph (OPEN-circulatory fluid of gastropods & bivalves) • Collected, pumped, and released DIRECTLY into the tissues (Cells are directly bathed WITHOUT vessels).
(3) Hemocoel (OPEN fluid-filled SPACE or blood-cavity) • Hemolymph drains BACK to the heart from hemocoel.
(A) Snails (eyes on the ENDS of modified tentacles) • Terrestrial (hermaphrodites, mantle cavity acts as a LUNG requiring MOIST air); Aquatic (separate sexes, respiration through GILLS)
Critical Thinking (1) Land snails are hermaphrodites. Of what advantage is this characteristic to the land snail?
(B) Other Gastropods (slugs, nudibranchs, pteropods) • Moisture is VITAL to exchange gases (slugs); Nudibranchs—naked gills; Pteropods—foot is modified for SWIMMING.
IV. Class Bivalvia (e.g., clams, mussels, scallops, etc…) • SESSILE lifestyle, 2-shells on a muscular hinge, and LACK a head region.
Critical Thinking (2) Marine clams and other aquatic mollusks reproduce by releasing sperm and eggs into the water. How might this process affect the reproductive success of these mollusks? Would you expect aquatic mollusks to release many sperm and eggs or only a few?
(1) Adductor Muscles • HINGE muscles open AND close shell for feeding, mating, and protection.
(A) Clams (marine (EXTERNAL) vs. freshwater (INTERNAL) reproduction • Bivalves that INHABIT mud or sand and feed as FILTER-feeders.
(1) Incurrent Siphon & Excurrent Siphon (Pair of hollow, fleshy tubes) • Draw IN and circulates OUT water after nutrients and wastes have been EXCHANGED.
Critical Thinking (3) Many clams have very long incurrent and excurrent siphons. For example, the siphons of a glam called the geoduck, Panope generosa, may exceed one meter in length. What do you suppose could be the adaptive advantage of such long siphons? (Keep in mind the habitat of most clams)
(B) Other Bivalves • Oysters (permanently sessile), scallops (move by BACKWARD propulsion), shipworm (ingests DRIFTWOOD, broken down by symbiotic gut bacteria).
Critical Thinking (4) Humans value pearls for their luster and color, features that are of no significance to an oyster. Furthermore, making a pearl consumes resources that an oyster could use for other purposes, such as strengthening its shell. Given these facts of what advantage is it to an oyster to manufacture a pearl?
V. Class Cephalopoda (“HEAD-foot”, MOST evolved mollusks) • Shells ARE flexible or LOST for a free-swimming, predatory lifestyle (e.g., octopuses, squids, cuttlefishes, and chambered nautiluses)
(1) Chromatophores • PIGMENT cells show a sudden change in COLOR, used with camouflage, mating, and intimidation.
(A) Squids (cephalopods with TEN tentacles) • 2 LONGER tentacles for capturing prey, 8 shorter to assist with feeding; can release a cloud of INK to deter predators. NOTE: The RECORD for the LARGEST invertebrate is the GIANT squid, Architeuthis, which can reach a length of 60 feet and way more than 3.5 tons)