THE MOLLUSCA 33 
(1) the antero-dorsal or cephalic, on which are borne most of the 
special sense-organs ; (2) the postero-dorsal or pallial, which forms 
a projecting fold around the body, and secretes on its external 
face a calcified cuticle or shell, and on its lower surface develops 
respiratory organs or ctenidia ; (3) the ventral or pedal, which is 
the organ of locomotion. 
6. A so-called ‘“‘veliger” or free trochosphere larva is nearly 
always present in embryonic development ; its preoral ciliated ring 
grows out to form a natatory velum, and at its formative pole 
there is a “ preconchylian invagination ” or shell-gland. 
V. Bronomics AND DISTRIBUTION. 
Molluscs are essentially aquatic animals, but the most varied 
modes of existence may occur, even among members of the same 
class. The majority are inhabitants of the sea; a few live in fresh 
water ; a single order of Gastropods and a few isolated members of 
the same group are adapted to a terrestrial life. They are repre- 
sented in the three great groups of aquatic organisms, namely, in 
the Benthos, comprising creeping or fixed animals inhabiting the 
depth of the sea; the Necton, comprising animals that swim 
actively and can make headway against the currents ; the Plankton, 
comprising animals that float passively and cannot contend with the 
currents. The first group includes the littoral and abyssal Molluscs, 
among which the Necton is also represented. The two other 
groups include the pelagic Mollusca, the Cephalopods belonging 
exclusively to the Necton, while the free-swimming Gastropods, 
and those which inhabit pelagic Algae, some isolated Lamelli- 
branchs (Planktomya), and the larvae of various groups belong to the 
Plankton. 
The Mollusca are distributed over the whole surface of the 
earth and in all latitudes. Terrestrial forms are found on the 
highest mountains—some Stylommatophora at a height of 15,000 
feet ; lacustrine forms (/imnaca) are found at a depth of 350 
fathoms. The pelagic forms are not only distributed over the 
surface of the sea, but may descend to a depth of 2600 fathoms 
without reaching the bottom. Abyssal Molluscs are found in all 
oceans extending to a depth of 2800 fathoms from the surface. 
As a rule, Molluscs are free-living animals, and crawl, swim, or 
burrow, but some are sedentary in adult life. Only a few 
Gastropods and Lamellibranchs are fixed to their habitat. Their 
modes of alimentation are various. Some are commensal with 
Ascidians, ¢.g. Modiolaria; some with Echinoderms, as Montacuta, 
Lepton, and Scioberetia ; some with Crustacea, Lepton squamosum and 
Ephippodonta, or with Sponges (Vulsella), or Annelids (Cochliolepis). 
Others again are ectoparasitic on Echinoderms, such are Zhyca and 
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