152 



Subkingdom VI. MOLLUSCA. 

 HETEKOGANGLIATA. 



Soft-bodied, unsegmented animals, often protected by a shell 

 The digestive system including stomach, intestine, and anus. 

 Keproduction by ova, rarely by gemmation. Hermaphrodite, 

 but generally the sexes on distinct individuals [dioecious], A 

 metamorphosis in nearly all. 



The nervous system generally consists of three pairs of ganglia, 

 giving off branches to the different parts ; but in Brachiopoda 

 and Tunicata there is only one ganglion. The heart has two or 

 more chambers, but in the Ascidians it is reduced to a simple 

 tube. The blood is generally colourless, and circulates in sinuses, 

 having no proper walls. Respiration is mostly effected by 

 branchiae, a specialized portion of the mantle ; there is one or, 

 most commonly, two on each side. 



The integument of the body, continuous or divided into two 

 lobes, is called the mantle [pallium]. From the ventral surface 

 projects a muscular disk or foot [podium], a modification of the 

 lower lip and an organ of locomotion, and generally the only 

 one, but it is sometimes absent. The foot is often divisible into 

 three parts pro-, ineso-, and metapodium ; it is the last that 

 sometimes secretes a calcareous or horny disk, known as the 

 " operculum." 



The mantle secretes the shell, which is rarely absent ; it may 

 be in one or two pieces [univalve or bivalve], rarely of many 

 pieces [multivalve] ; in a few instances it is internal. The inner 

 layer of the shell is sometimes nacreous in its texture ; its colour 

 is due to finely sculptured lines. A rudimentary shell is frequently 

 present whilst the young mollusk is still in the egg, and it has 

 even been observed in forms which have no shell in the adult 

 state. 



In certain classes the mouth is provided with an organ [radula 

 or odontophore], often, but erroneously, called the tongue. It is 

 armed with teeth (in the large garden-slug, Limax maximus, they 

 amount to 26000) and is an important part in classification. It 

 is found in Pteropoda, Gastropoda, and Cephalopoda. 



The embryo of the Mollusca ordinarily passes through three 

 stages: (1) the Gastrul&-atage, including the earlier Morula; 

 (2) the TrochospJiGra-tiagQ, when the embryo is girdled with a 



