PROGRESSION AND RETROGRESSION. 183 



The history of the inner king is indicated by fishes and amphibians. 

 The history of the outer king is indicated in these naked moUusks. 



EoHs, which shows us the beginning of a Uver, or perhaps tlie 

 last stage of its reduction, seems to be prehistoric as to tlie gill. 

 One part of the surface absorbs oxygen as well as another. If we 

 leave the beach and the Eolids for mid-ocean and the Pteropods, we 

 shall find the first shadowing forth of a gill. In the Pteropod one 

 part of the skin is a little more vascular than the rest, and on this part 

 the blood is more freely oxidized. Now " respiratory activity," as 

 Spencer has shown, "aids in the development of respiratory append- 

 iiges." A larger and larger surface is exposed to the water, and this 

 larger surface, developed partly by Natural Selection, and partly by 

 respiratory activity itself, is attained in multitudinous branchings of 

 the mimic tree, and deep sinuosities of the mimic leaf. 



But in Doris, which represents a great advance in gill development 

 over a Pteropod, the gill is still imperfect, and as a respiratory organ 

 it is supplemented by the creeping disk. In Aplysia the gill is car- 

 ried up to perfection and aerates all the blood. 



In the evolution of an organ we have hints as to the evolution of a 

 species. 



.No interest can attach to such low forms of life as the Eolids un- 

 less they teach something of the methods of Nature in originating 

 species. Readers of The Popular Science Monthxy will not give 

 their attention to mere description or anecdote. Facts they know do 

 not pass into science until fertilized by ideas. We shall return to 

 Eolis and its family through a study of forms which the eye, not 

 aided by the knife, would report as far removed from them. 



A raollusk is a soft, fleshy, sac-like body, with a mantle (paUium) 

 extending from the back in two folds, right and left, around the sides. 

 In the Bryozoan (moss-animal), whose reticulated coral incrusts many 

 shells and sea-weeds, the molluscan type reaches down almost to the 

 polyp. The Bryozoan has a cylindrical body with a tentacular crown. 

 Structurally it is a mollusk, morphologically a polyp. It would seem 

 to be a case in the organic world analogous to that in the inorganic, 

 in which a small portion of a mineral, in crystallizing, forces a large 

 portion of a foreign mineral into its own crystalline form and masks 

 the structure under the shape. 



The mantle performs important functions, and it will guide us 

 along a series of transformations. Suppose that the two folds cohere 

 along their edges. The mantle would then become a kind of sac, in- 

 closing the body. If we call it a tunic, v/e might say that the animal 

 is wrapped in its tunic, and this cohei'ing of the tunic-folds would 

 bring us to the order of Tunicata. 



If we put the dredge down fathoms deep into the sea, it may bring 

 from the bottom a Clavelina, most beautiful of Tunicates. In shape it 

 is a pitcher without handle, an inch high, tapering down to a slender 



