MOLLUSCA (III): DIVERSITY-Gastropoda (Mollusca study images;
|
a) primitively were rasping grazers of epibenthos, but feeding modes are remarkably diverse and often reflected in radular form: simple rasping, shredding, piercing, drilling, harpooning; some even filter feed!
b) single, often coiled shell; body attaches with 1 columellar muscle
c) adapted for a creeping, epibenthic life mode (but many burrowing and some swimming forms) well-developed head and sense organs
d) gill form varies greatly: 2-bipectinate -> 1-unipectinate -> none
e) have a large, creeping foot; a horny operculum functions like protective trap door (in marine snails and some sea slugs)
-process: posterior end rotates over head in two phases; first phase is muscular (asymmetric pedal retractor muscles), second is morphogenetic
-consequences: mantle cavity, openings to anus and excretory systems become anterior; many body asymmetries (loss of 1 gill in some, twisted nervous system, unequal gonads and kidneys)
-adaptive significance remains a puzzle; likely benefits the larvae (foot and protective operculum withdraw into larval shell last for better defense against micro-predators)
-Prosobranchia (marine shelled snails) large shell, anterior mantle cavity, 1 or 2 ctenidia- some unipectinate, operculum, separate sexes
-Opisthobranchia (sea slugs) shell and mantle cavity reduced or absent; ctenidia and operculum absent in most; hermaphroditic
-Pulmonata (land snails and slugs) shell present or absent; mantle cavity is a lung; ctenidia and operculum absent in all; hermaphroditic; ancestor was likely marine, not freshwater
Copyright (c) 2007 by A. Richard Palmer. All rights reserved.
(revised Jan. 28, 2007)