MOLLUSCA OF NEW ZEALAND. 



PHYLUM MOLLUSCA, Cuvier. 



THE Mollusca are originally bilateral organisms in which signs of 

 primitive segmentation are no longer evident. 



They possess a well-developed coelom (gonad and pericardium), 

 enteron, and hsemocoel, quite distinct from one another. 



The alimentary tract exhibits (or has lost) a radular sac in its 

 anterior part. 



The nervous system consists of a peri-cesophageal ring, whose 

 supra-cesophageal (or dorsal) moiety is the cerebral commissure, and 

 the infra-oesophageal (or ventral) moiety is the labial commissure. 

 The former gives off chiefly sensorial nerves, the latter nerves to the 

 digestive tract. From their union two nervous cords arise on each side 

 a dorsal or pallial, and a ventral or pedal ; from the former arise the 

 visceral nerves, whose main trunks are frequently joined together under 

 the digestive canal to form the infra-intestinal visceral commissure. 



The general body-wall is differentiated into three regions : (1) the 

 antero-dorsal or cephalic, on which are borne most of the special 

 sense-organs, called the head ; (2) the postero-dorsal or pallial the 

 mantle which forms a projecting fold around the body, and secretes 

 on its external face a calcined cuticle or shell, and on its lower sur- 

 face develops respiratory organs or ctenidia ; (3) the ventral or 

 pedal the foot which is the organ of locomotion. 



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 " pre- 

 conchylian invagination," or shell-gland. 



The Mollusca are divided into six classes, which may be inter- 

 preted by the following mostly well-known animals : (1) the mail- 

 shell, or Chiton ; (2) the free and floating hyaline pteropods, with 

 wing-like lobes as organs of motion ; (3) the snail and slug ; (4) the 

 tusk-shell, or Dentalium ; (5) the mussel ; (6) the cuttlefish. 



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 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 ft. ; lacustrine forms are found at a depth of 



1 Moll. N Z. 



