DIVISION OR TYPE III. : MOLLUSCA. 



Bilaterally symmetrical, soft-bodied animals, without segmented limbs, 

 with a ventrally-placed locomotor organ (foot), and a cutaneous fold 

 (mantle) above the foot, which encloses the viscera, covers the respiratory 

 organs (branchiae, or lungs), and, as a rule, secretes a calcareous shell. 



Ground-plan of Structure. — The Mollusca, which form the large 

 and widely-branched third division of the animal kingdom, are, like 

 vertebrates and arthropods, bilaterally symmetrical animals (see Part I., 

 p. 2; this symmetry, however, disappears in the majority of the Gas- 

 tropoda). They, however, neither possess an internal skeleton like 

 vertebrates, nor an external one like arthropods. Their body, in fact, is 

 of a soft nature (Mollusca = soft-bodied animal ; Lat. mollis = soft), and is 

 covered by a slimy integument. Hence these animals are for the most part 

 inhabitants of the water, and, being wholly or partly supported by the 

 surrounding water, neither require a firm and solidly-constructed body 

 like land animals, nor an external body-covering for the purpose of 

 preventing an excessive evaporation of water into the surrounding air 

 (see Eoman or vineyard snail). 



Although the three principal types of the Mollusca — viz., the snail, 

 the mussel, and the cuttle-fish — are of widely-different shapes, certain 

 essential and constantly-repeated features may be recognised in their plan 

 of structure. Since the nomenclature of the principal body parts has 

 been borrowed from that of the land snails, we shall here confine our- 

 selves to a detailed consideration of this group. These animals, when 

 creeping extended along the ground, display on their lower or ventral 

 surface a large muscular portion, by means of which locomotion is 

 effected. It therefore functionally represents the limbs of vertebrates 

 and arthropods, and is accordingly described as the foot. It supports 

 the body- of the animal, which encloses the viscera (intestine, liver, 

 heart, and other organs, and in front is continued into a more or less 

 distinctly separated head. The skin which envelops the body forms 

 above the foot a dependent fold, which covers in a hollow chamber. 



