366 THE POPULAR SCIENCE MONTHLY. 



day and examine it through and through for its fauna, and whenever 

 I did this I found many notable cases of mimicry. The color of the 

 younger plants is a yellowish green, while the older stalks are of a 

 more or less dark brown. A luxuriant animal life flourishes on the 

 stems, leaves, and air-vessela of the Sargasso-weed. Little actiniae, 

 sometimes of a light, sometimes of a dark-brown color, were very 

 numerous on the plants I most closely examined ; often so thick as to 

 completely cover the stems. On the same plants, I also found numer- 

 ous specimens of small, naked snails. These minute gasteropods, a 

 centimetre or a centimetre and a half long, bore on their backs numer- 

 ous retractile tentacles, arranged in cross-rows at various intervals. 

 In color, they were of various shades of brown, like the actiniae ; when 

 they drew themselves up so that the tentacles stood thickly together, 

 they so much resembled the actiniae that it would be a matter of 

 difficulty to a person not acquainted with both animals to tell them 

 apart. Another snail, whose tentacles were arranged in rows along 

 each side of the back, was still more difficult to distinguish when any 

 danger threatened it from the actiniae. Of what use can the resem- 

 blance to the actinia3 be to the little mollusks ? They are, it is true, 

 great eaters of the actiniai, for I have seen one of them devour four or 

 five of those animals in an hour ; but it does not appear that their 

 access to them is greatly facilitated by the resemblance, for the 

 actiniae are so confined by the limitation of their movements as to be 

 unable, in any case, to escape their more facile enemies. We are, there- 

 fore, reduced to consider the likeness a case of mimicry for the protec- 

 tion of the snails against animals which pursue them, but avoid the 

 actiniae, whose nettle-cells are by no means pleasant morsels. But as 

 I have not been able to discover what special enemies the snails have, 

 and whether they really dislike the actiniae, my attempted explanation 

 must remain an attempt, to be confirmed or disproved by some future 

 observer in the Sargasso Sea, who can begin where I have had to leave 

 off. Other cases of mimicry on the part of mollusks have come under ob- 

 servation. According to Dr. H. von Ihering, the Chromodoris gracilis 

 lives associated with a sponge (Suberites), and is colored like it, blue. 



On the same sea-weed I found other larger mollusks, which, not 

 resembling other animals, so strikingly resemble the forms of the 

 stems and leaves of the plants that it is extremely difficult to find 

 them in the tangle of brush. They have developed flaps all around 

 their bodies, before and behind, and on either side, the edges of which 

 are irregularly serrated, with the tips of the serratures of a brown tint 

 like the older alga-stems. The surface of the flaps, and of a part of 

 the rest of the body, is beset with numerous small similarly brown- 

 tipped teeth, while the color of the animal as a whole is olive-green, 

 like that of the plant in which it lives. 



Moritz Wagner regards the phenomena of mimicry as the conse- 

 quence of an innate caution in the animal, that causes it to choose those 



