2. MR MACDONALD ON THE ANATOMY 



The zoological characters of these three families and their genera are given in 

 the following Table of Classification : — 



Ueteropoda. 



I. Gymnosomata* (Firolidae). Animal wholly naked or without a shell. 



1. With slender tentaeula, and destitute of true branchiae. Visceral mass near the root 



of the filiform process of the metapodium, — Fh-oloidcs. 



2. Witli rudimentary or no tentaeula, hut furnished with true branchiae. Visceral mass con- 



siderably in advance of the base of the filiform process of the metapodium, — Firola. 



II. Thecosomata inoperculata (Carinariidae). Animal in great part naked, but having the 

 visceral mass protected by a shell. 



1, Shell corneous, with an involute nucleus. Swimming-plate nearly opposite the ^^sceral 



mass. Metapodium with filiform appendage, — Cardiapoda. 



2. Shell calcareous, with spiral nucleus. Swimming-plate considerably in advance of the 



visceral mass. Metapodium laterally compressed, without filiform appendage, — 

 Carinaria. 



III. Thecosomata operculata (Atlantidae). 



1. Shell corneous, with an involute nucleus. Operculum suhtrigonal, with small lateral 



subapical nucleus, — Oxygyrus. 



2. Shell calcareous, with a spiral nucleus. Operculum oval, with a large median sub- 



apical nucleus, — Atlanta. 



I have thus far anticipated myself in the construction of the preceding table, 

 but as it affords a bird's-eye view of the subject, it will be found convenient for 

 reference as occasion may require. 



General Outline of the Order. 



The Heteropoda are pre-eminently distinguished by the laterally compressed 

 and fin-like configuration of the body of the foot and propodium ; the rudimen- 

 tary state of the creeping disc {mesopodium), and the great length of the opercu- 

 ligerous lobe {inetapodium), or its homologue, often continued into a kind of caudal 

 appendage. 



The remarkable transparency of the tissues of these animals reveals a great 

 part of their internal structure to the anatomist without dissection, which is 

 often a matter of great difficulty, arising from the same circumstance. They are 

 all furnished with a cylindrical proboscis- like muzzle, and a well marked neck, 

 large, mobile, and singularly beautiful eyes, lying in socket-like spaces, though 

 invested with the common integument at the posterior part of the base of simple 

 conical tentaeula, except in the true Firolce, in which the tentaeula are absent, 

 or at most very rudimentary. 



The auditory sacs in all contain single spherical otoliths, increasing by exo- 

 genous layers, and revolving like planets on their axes, by the action of vibratile 

 cilia lining the sacs. The large size of the lenses of the eyes, as compared with 

 that of the otoliths, affords a character, which, although of a relative nature, is 



* I have made use of the convenient terms Gymnosomata and Thecosomata in a similar sense 

 to that in which De Blainvtlle applied them to the Pteropoda. 



