Chap. VI.] GlLLS OF MOLLUSCA. 221 



gill filaments, and which are now the means of passage 

 for the circulatory medium. With this considerable 

 alteration in structure there would seem to have come 

 some change of function, and Peck is probably right 

 in believing that the ciliated gill plates of the Lamelli- 

 branchs, by causing a constant current of water to pass 

 over the animal, are as important aids to their nutrient 

 as to their respiratory processes. The spaces between 

 the gill filaments are sometimes used as incubatory 

 pouches, in which the dlochidia, or young of the 

 fresh- water mussel, lie during the early days of their 

 development. 



The branchial outgrowths (ctenidia) of the 

 Cephalophora are. on the whole, simpler in structure 

 than in the more highly developed members of the 

 Lamellibranchiate group ; in most of the lowest forms 

 they are arranged in a circle, as in Neomenia, where 

 there is an oblong circlet of thirty filiform tubules, the 

 Chiton, or the limpet ; in Haliotis and others they 

 are distinctly bilateral, and in the Aiiisobranchiata 

 the left gill is, OMdng to the torsion of the body, much 

 smaller than the right ; in others, as the Hetero- 

 poda, the left gill disappears altogether, and in some 

 the right gill has only one lamella developed ; this is 

 the so-called semi-pinnate gill. In other Gastro- 

 pods, such as Lymnaeus, or Helix, the gills disappear 

 altogether, and a so-called lung is developed. (See 

 page 228.) In a number of naked Pteropoda re- 

 spiration becomes vague by a process of adaptative 

 degeneration ; or, in other words, the whole surface of 

 the body takes on the function of the lost gills. Among 

 the Cephalopoda the Tetrabranchiata (Nautilus) 

 have two pairs, and the Dibranchiata (Octopus, Loligo) 

 one pair of well-developed gills. 



In the great majority of cases the gills of the 

 Mollusca are covered in by the mantle, and come, 

 therefore, to lie in a branchial chamber; into 



