MOLLUSOA OF PLYMOUTH. 419 



The structure of the foot of Oscanius and its habits of loco- 

 motion are of interest as indicating the way in which the lateral 

 folds of the foot (epipodia) of Aplysia, Lohiger, and other Opistho- 

 branchs have probably arisen. As Cuvier originally pointed out, 

 these lateral folds in reality correspond to the lateral portions of 

 the undifferentiated foot of such forms as Scaphander, Oscanius, 

 and Haminea. In Oscanius the sides of the foot are frequently 

 folded over the body of the animal when at rest^ and this habit is 

 still more marked in Haminea. Now the animals are found to live 

 on muddy bottoms^ and a broad flexible expanse of foot is obviously 

 advantageous for gliding over such surfaces (cf. Alderia niodesta, 

 which also creeps upon mud). But the habits of Aplysia are 

 different. Aplysia lives upon algte, and for creeping over the 

 narrow stems and fronds of seaweeds a wide plantar surface would 

 be not only unnecessary but disadvantageous ; so we find that the 

 median portion of the originally broad foot has become specialised 

 for creeping purposes, while the lateral portions no longer form part 

 of the plantar surface, but arise from the vertical sides of the median 

 portion, and retain only their power of flapping for the purpose of 

 natation. The series of forms illustrating the evolution of the 

 lateral folds of Aplysia is so complete as to leave no doubt about 

 the truth of this view ; but at the same time it becomes almost 

 impossible^ to regard the epipodia of Opisthobranchs and those of 

 Haliotis and the .lower Prosobranchs as strictly homologous. On 

 this account Von Jhering has proposed for the folds of Aplysia the 

 term " parapodia/' which has been adopted by Pelseneer, and in 

 part by Vayssiere. Professor Herdman^ still regards the homology 

 as possible, and therefore retains the name " epipodia ^' for the lateral 

 folds of Opisthobranchs ; but he justly objects to the term " para- 

 podia,'' as being " already appropriated by a totally different 

 structure in another group of animals/^ Perhaps the term " pleuro- 

 podia " would at the same time be free from this objection, and also 

 prevent confusion with the epipodia of the Rhipidoglossa. 



This species is well known to secrete from its general body- 

 surface a fluid containing sulphuric acid, which reddens blue litmus 

 strongly. As Bateson^ has shown that food otherwise palatable is 

 refused by fishes generally when it " has been soaked for a few 

 minutes in dilute acids," there can be no doubt that this secretion 

 is a great means of protection to the species from the attacks of 

 fishes. I have tasted this fluid, and it is strongly acid ; but I have 



1 See Pelseneer, Sur VEpipodium des Mollusques, Bull. Sci. Fr. Belg., 1888, p. 192, &c. 

 "^ Herdman and Clubb, Third Report on the Nudibranchiata, Trans. Liverpool Biol. Soc, 

 iv, p. 147. 



3 Bateson, The Sense Organs and Perceptions oj Fishes, this Journal, N. S., vol. i, p. 247. 



