GASTEROPODA HETEROPODA. 351 



ORDER V. 



HETEROPODA, Lam.(i) 



The Heteropoda are distinguished by their foot, which, in- 

 stead of forming a horizontal disk, is compressed into a vertical 

 muscular lamina, which they use as a fin, and on the edge of 

 which, in several species, is a dilatation forming a hollow cone, 

 that represents the disk of the other orders. Their branchiae, 

 composed of plumiform lobes, are situated on the hind part of 

 the back, directed forwards, and immediately in their rear 

 are the heart and a small liver, with part of the viscera and 

 the internal organs of generation. Their body, a gelatinous 

 and transparent substance lined with a muscular layer, is elon- 

 gated and usually terminated by a compressed tail. There is 

 a muscular mass belonging to the mouth, and a tongue fur- 

 nished with little hooks; the oesophagus is very long; their 

 stomach thin ; two prominent tubes on the right side of the 

 visceral bundle afford a passage to the faeces, semen and ova. 

 They usually swim on their back with the foot upwards(2). 

 They have the faculty of distending their body by filling it 

 with water, in a way not well understood. Forskahl comprised 

 them all in his genus 



(1) M. de Blainville makes a family of the Hetehopoda, which he names Nec- 

 TOPODA, and unites them in his order of the Nucieobranchiata with another 

 family that he calls Pteropoda, and which, of all my Pteropoda, only includes the 

 Limacina. He joins the Argoiiauta with it, on account of some conjecture, of 

 which I am ignorant. 



(2) This mode of natation induced Peron to think that the natatory blade was 

 on the back, and the heart and branchis under the belly, and has given rise to 

 many errors as respects the place of these animals. A simple inspection of their 

 nervous system led me to suppose, in my Memoirs on the Mollusca, that they 

 were analogous to the Gasteropoda. A more exact anatomical investigation, made 

 since then, with that given by M. Poll in his vol. Ill, fully confirms my supposition. 

 The fact is, that there is but little difference between the Heteropoda and the 

 Tectibranchiata, notwithstanding which, M. Laurillard believes their sexes to be 

 separated. 



