Organs employed in the Classification of the Mollusca. 391 



These two series prove not only that the size and form of the 

 gills stand in connexion with the development of the shell, but 

 that the divisions Opisthobranchiata and Prosobranchiata, as 

 originally proposed, are not natural ; this has been indicated by 

 Messrs. Huxley and Macdonald. 



The two kinds of respiratory organs indicate only relative 

 superiority and inferiority, but not limits of systematic divisions. 

 Thus, among Vertebrata, lungs and gills meet in a family of the 

 Batrachians. Among the Mollusca, it is probably in Auricula 

 and Oheliscus that the two kinds of respiratory organs are found 

 to meet. 



It has been generally understood that neither the locomotive 

 nor the respiratory organs offer characters for the primary divi- 

 sions; most authors have therefore divided the Mollusca, ac- 

 cording to the presence or absence of a distinct head, into 

 Cephalophora and Acephala. As several Gasteropoda [e.g. The- 

 cosomata) properly have not a distinct head, Prof. Loven has 

 changed the names to Glossophora and Aglossa. Cephaliza- 

 tion, as explained by Prof. Dana, is, no doubt, of importance as 

 a character indicating relative superiority and inferiority, but is 

 not sufficient for natural divisions. Thus, according to this 

 principle, the Vertebrata would only be divided into two classes, 

 1, Cephalophora with a distinct head — Mammalia, Birds, and 

 Reptiles ; 2. Acephala, with the head united to the thorax, con- 

 taining the Fishes only. 



My study of the Mollusca for about twenty years enables me 

 to state that the heart and generative organs offer characters of 

 a much higher systematic value than is generally beheved. It 

 is perhaps somewhat hazardous to compare the organs of the 

 lower animals with those of the higher; but it does not seem 

 probable that organs which have no systematic value in the 

 higher can have it in the lower animals. 



The accompanying synopsis of the Mollusca is chiefly founded 

 on the intromittent male organ, which seems to me to be the 

 best indicator of the sensibility of the nervous system, and con- 

 sequently of the relative systematic rank of the animal. Thus 

 the lowest class of Vertebrata (the Fishes) wants an intromittent 

 male organ, although the sexes, with few exceptions, are sepa- 

 rate; there exists, consequently, no copulation, but impregna- 

 tion takes place as among plants. In the Plagiostoms the pos- 

 terior locomotive organs of the male are changed into conduits 

 for the sperm*, like the hectocotylized arm of the Cephalopods. 

 In the Batrachians the anterior locomotive organs are used as 

 prehensile organs during the pseudo-copulation ; but a true 



* Steenstrup, Om Hectacotyldannelsen, p. 26, Kgl. Danske Videnska- 

 bemes Selskabs Skrifter, 5. Rsekke, 4. Bind. 



