320 MOLtUSCOtJS ANIMALS. 



in the Journal has led to their substance being put together in the 

 present form. "What is here offered is but the introduction to inqui- 

 ries respecting the true arrangement and affinities of the principal 

 groups which have engaged the author's attention, and the results of 

 which, if opportunity is allowed, he hopes to bring before the pubhc 

 in one or two following papers. 



MoLLUSCA, (soft bodied animals) is one of the four sub-kingdoms 

 or great branches of the Animal kingdom established by Cuvier, and 

 adopted by most modern naturalists, many of them, however, recog- 

 nizing a fifth sub-kingdom, named Protozoa : We will first consider 

 the characteristics and the true limits of Mollusca ; then its relations 

 to the other sub-kingdoms, and afterwards the classes into which it is 

 properly divided, with their sub-divisions. The treatment of the 

 subject is in harmony with the views, already defended by the author 

 respecting the classification of organised beings, but' so far is he from 

 being conscious of any straining of facts to produce this harmony, 

 that it is to him only an additional illustration of a great universal 

 law, presenting itself at once to the view of the careful observer. 



Mollusca are unsegmented animals of a sac-like figure, the viscera 

 being enclosed in a common envelope called the mantle ; with a 

 nervous system of one or several ganglia, when more than one, 

 unsymmetrically disposed ; and with motory apparatus formed by 

 various modifications of the parts of a single central foot. 



When we consider that both Vertebrate and Articulate animals are 

 segmented internally or externally and have their nervous ganglia, so 

 far as developed, doubly serial, belonging to the several segments ; 

 that Radiate animals consist of a set of merosomes in each of which 

 the same functions are performed, whirled round a common axis, in 

 which a common alimentary sac or canal is placed, and with equal 

 provision of nervous power for each raerosome ; whilst Protozoa have 

 ;a sarcode body, with no distinct nervous system, and the lowest 

 differentiation of functions, but without either serial segmentation or 

 any tendency to the assemblage of distinct approximately equal 

 merosomes to form a common body belonging to a distinct animal, 

 ^ve shall see that the definition given above, whilst applicable, to the 

 whole Molluscan series, abundantly distinguishes it from all other 

 animals. 



Respecting the true limits of the sub -kingdom Mollusca, there may 

 be three points which claim some passing notice, though only one of 



