682 MOLLUSCA. 



missures. The most important ganglia are (i) a pair of " cerebral " 

 ganglia, placed above the gullet; (2) a pair of "pedal" ganglia 

 placed below the gullet, and supplying the foot and adjacent parts ; 

 and (3) a group of " visceral " ganglia placed towards the posterior 

 part of the body. 



Reproductive organs are always present, and the sexes may be 

 distinct or united. There is usually a well-marked metamorphosis 

 in development, and the embryo very commonly passes through a 

 " trochosphere " stage, in which it swims about actively by means 

 of a circlet of cilia placed in front of the mouth, and closely re- 

 sembles the larva of many Annelides. The young Mollusc pos- 

 sesses a glandular involution of the dorsal integument, which pro- 

 duces an embryonic shell, and this may either be cast off as de- 

 velopment proceeds, or may remain permanently as part of the 

 shell of the adult. In the Spiral Gastropods the embryonic shell 

 or " nucleus " is placed at the apex of the permanent shell, whereas 

 in the Bivalves it is situated at the beak or " umbo." 



The Molluscs very generally develop an integumentary skeleton 

 in the form of a " shell," which in all forms, except in the Argonaut, 

 is secreted by the mantle. In chemical composition the shell con- 

 sists of carbonate of lime disseminated throughout an organic 

 matrix. The atomic arrangement of the carbonate of lime differs 

 in different cases, some Molluscs having the shell wholly composed 

 of calcite, while in others it is wholly composed of aragonite, and 

 in many forms it consists of an inner layer of aragonite and an 

 outer layer of calcite. Very generally the outer surface of the shell 

 is covered by an easily recognisable horny external layer (the " epi- 

 dermis " or " periostracum "), but this may be exceedingly thin, or 

 may disappear altogether in the course of growth. Some of the 

 more important points in the microscopic structure of the shell will 

 be briefly noticed in dealing with the different groups of Molluscs. 

 While the presence of a shell is very characteristic of the Molluscs, 

 there are many so-called " naked " forms, in which the adult is 

 either totally devoid of a shell, or possesses only a rudimentary 

 shell enclosed in the substance of the mantle. 



The Mollusca may be roughly divided into two great sections, 

 respectively termed the Acephala and the Encephala (or Cephalo- 

 phora), characterised by the absence or presence of a distinctly 

 differentiated head. The headless, or Acephalous, Molluscs corre- 

 spond to the class Lamellibranchiata ; also distinguished, at first 

 sight, by the possession of a bivalve shell. The Encephalous 

 Molluscs are more highly organised, and are divided into the two 

 principal classes of the Gastropoda (including the Pteropodd) and 

 the Cephalopoda, with which may be associated the two small and 

 aberrant groups of the Polyplacophora (Chitons) and Scaphopoda 



