ON THE MORPHOLOGY OF THE CEPHALOUS ArOLLUSCA 153 



with regard to the archetypal molluscous form, and the mode in which 

 (if such an archetype exist) it becomes modified in the different 

 secondary types. So far as our knowledge goes among other forms of 

 animal life, we invariably find that, whatever the subsequent variations 

 and aberrations, the primordial embryonic form has its parts arranged 

 symmetrically about a given axis. 



No one imagines the Pleuronectidae belong to an asymmetrical type 

 because they are asymmetrical in their adult shape, and yet there is 

 no stronger evidence for the very common assertion that the typical 

 form of the Mollusca is spiral or asymmetrical.^ 



This unsatisfactory state of our knowledge appears to me to result 

 from two causes ; — first, from the want of a clear and definite concep- 

 tion of the fundamental varieties of molluscous structure, and of the 

 nature of the changes in the relations of parts which constitute those 

 varieties ; and secondly, from the want of a due regard to the facts 

 presented by the development of the different families, which must 

 stand in the relation of cause to the varieties of form. 



Now in order to the former end (the obtaining of a definite concep- 

 tion of the varieties of molluscous form), I propose to set forth the 

 structure of certain Heteropoda and Pteropoda ; pelagic animals so 

 transparent, that a perfect knowledge of the arrangement of their 

 parts may be arrived at by simple inspection, without so much as 

 interrupting a beat of their heart. 



Afterwards I shall inquire how far the known laws of development 

 account for these forms, and thence of what archetypal form they may 

 be supposed to be modifications. 



PART I. 



I. Anatomy of Firolo'ides (Plate II. [Plate 17] fig. i). 



The species of Firola which I examined appears to be identical 

 with the Firolo'ides Desmarestii of MM. Eydoux and Souleyet.- 



The animal may be described as a transparent cylinder about an 

 inch long, and so generally colourless as to be hardly distinguishable 

 in the water, except by the incessant flapping of its flattened ventral 

 appendage (//). 



The only parts which present any colour are the buccal mass, which 

 is brownish ; the eyes, almost black, and the mass of the liver, A\'hich 

 is brownish-green ; further, the anus has a pinkish tint. 



' See Von Baer, Nova Acta Acad. Nat. Cur. vol. xviii. p. 753. 



2 Figured by them in their beautiful plates illustrative of the Zoology of the Voyage of the 

 Bonite. 



