THE MOLLUSC A 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. BIONOMICS 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 (PlanUomya), 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 (Limnaea) 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 

 burro\v, 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, e.g. Modiolaria ; some with Echinoderms, as Montacuta, 

 Lepton, aa Scioberetia ; some with Crustacea, Lepton squamosnm and 

 Ephippodonta, or with Sponges (Vulsella), or Annelids (Cochliolepis). 

 Others again are ectoparasitic on Echinoderms, such are TJiyca and 



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