i82 THE POPULAR SCIENCE MONTHLY. 



Inimp. Aplysia has a toig of stomaclis, and, what is strange, the teeth 

 are not inserted in the mouth, but in one of the stomachs. In Aplysia, 

 the liver is better defined than in Doris, and the leaf-like gills aerate 

 blood for the whole body. 



The classification of these naked mollusks will be as obvioiis now 

 to the reader as to the observer. 



In Eolis no liver, but a few bile-cells representing its rudiment, or 

 vestige; no lung, every part of the surface respiring for itself; no 

 well-diflerentiated stomach, but an arborescent intestinal tube. 



Fig. 2. Doris plumulata. 



In Doris (sea-leraon), a liver; respiratory organs in the guise of 

 crown, or star, or leaf, or tufts of sea-weed, organs which serve the 

 liver only ; a stomach. 



In Aplysia (sea-hare), a better liver, respiratory organs in the form 

 of leaves, organs which serve the whole body ; many stomachs. 



Eolis stands lowest, Aplysia highest. The series is suggestive of 

 the history of organs, if not of species. It invites special attention to 

 the lung. 



In all marine animals except Cetacea, either the entire outer sur- 

 face absorbs oxygen and exhales carbonic acid, or part of this surface 

 has been diflTerentiated for the function of respiration. In all mammals, 

 and birds, and mature reptiles, part of an inner tissue has been difler- 

 entiated and set apart for the function of. respiration. External re- 

 spiratory organs rise from the skin. Internal respiratory organs rise 

 from the skin of the throat. Internal respiratory organs exist in the 

 fish as a rudiment. External respiratory organs apj^ear in embryotic 

 mammals as vestiges. 



The inner lung begins as a little hollow bud on the throat. This 

 bud pushes out another and another, and so on till by continuous bud- 

 ding it becomes a tree-like growth, interlaced with blood-vessels. 

 Let such a bud start from the outer surface, on the back. It will be- 

 come, according to the mode of secondary budding, a little tree, or 

 leaf, or flower of blood-vessels and vascular tissue such a lung as 

 adorns the back of Doris. 



