PLOTOSUS 147 



and this was followed by a number of buccal in- 

 spirations without the gill-covers being reflected. 

 The raising of the gill-covers coincides with the 

 taking of a deep breath. I counted first 32 in- 

 spirations and 13 expirations in a minute, then 

 32 inspirations to 10 expirations ; a third count- 

 ing gave 30 and n, a fourth 29 and 12. The 

 expirations were not evenly distributed, some- 

 times 5 or 6 occurring together. These observa- 

 tions show the great physiological importance of 

 the opercular membrane whose systematic worth 

 is sometimes regarded as nil. 



The typical Molluscan gill was named the 

 ctenidium by Lankester 1 in order to fix its 

 morphological independence and to distinguish 

 it from other gills of invertebrates such as the 

 Annelid parapodial gill, the Crustacean arthro- 

 podial gill, the Limuloid gill-book, and others, 

 as well as from secondary Molluscan pallial gills, 

 as in the Prosobranchiate genus Patella (limpet) 

 and in the Opisthobranchiate genera Phyllidia 

 and Pleurophyllidia. Thus the organs of re- 

 spiration which take the form of cutaneous gills 

 are not homologous throughout ; in other words, 

 they are not monophyletic but polyphyletic in 

 nature and origin ; but the particular structure 

 known as the ctenidium with which a special 

 sensory apparatus, the osphradium and its nerve- 

 supply, is associated, is homologous throughout 



1 Zoological Articles (Encyc. Brit.\ Edinburgh, 1891. 



