INSTINCT OF MOLLUSCA. 177 



sources a few mollusca add the protection which a disguise 

 of extraneous matter may give them. Thus, a very few 

 Ascidians invest their outer tunic with a coat of sand and 

 gravel, and become indistinguishable from the surrounding 

 ground.* The Gastrochaena conceals itself in a case of 

 agglutinated gravel ; and the Carrier Shells (Phoridae) attach 

 to the outer surface of their shell, as it enlarges in size, 

 stones, fragments of other shells, coral, and other marine 

 substances, whence one has been called the " Conchologist," 

 and another the " Mineralogist." Some species have this 

 habit only in an early stage ; others retain it during the 

 whole period of their life.-j" 



After a somewhat similar fashion, several land-snails 

 cover themselves with a coating of extraneous materials, 

 adapted to the different situations in which they are found, 

 so as to be detected with difficulty. The Pupa avena, 

 Drap., when living on rocks, clothes itself with a slight 

 layer of white powder ; but when it descends to the ground, 

 its white dress is exchanged for one of a yellowish or 

 greyish earth ; and, according to Draparnaud, the greater 

 number of the Pupae and Clausiliae have similar habits. J 

 Of the Bulimus obscurus (Fig. 32) Mr. Sheppard says, " If 

 its abode be upon the trunk of a tree covered with lichens, 

 then is the epidermis so constructed as to cause the shell to 

 resemble a little knot on the bark, covered with 

 Fig. 32. such substances. If on a smooth tree, from whose 

 bark issue small sessile buds, as is frequently the 

 case, it will pass off very well for one of them; 

 and on a dry bank, or the lower part of the body 

 of a tree splashed with mud, its appearance will be 

 that of a little misshapen pointed piece of dirt." § This is an 



firmandam, contra ventorum impetum ; alias vero rationes nescio. Actio 

 sane mirabilis." — Conchyl. BivuLv. Exert. Anat. tert. auct. p. xvii. 



* Of Ascidia conchilega Brugiere writes, " Its surface is rough and thinly 

 covered with long hairs, which are not easily ohserved, because of a singular 

 faculty that the animal has of making an external envelope of fragments of 

 shells, gravel, and of the vegetable and animal remains found about it : these 

 fragments are so attached to the body that they cannot be torn away; but it 

 appears that the creature itself parts with them and abandons them when 

 this is necessary to its safety." — Encyclop. Mctk. Vers, i. 148. See, also, 

 Kirby's Bridgew. Treat, i. 229. + Gray in Syn. Brit. Mus. (1842) G4. 



X Moll. Ter. et Fluv. de la France, 17. 



§ Lin. Trans, xiv. 1 16. According to Mr. Jeffreys, the Pupa secale 

 exhibits the same instinct when young. " This seems a provisional defence 

 to the animal until the teeth of the aperture are completely formed, when it 

 divests itself of its coat by rubbing the shell against extraneous substances ; 

 and it is one of the many and various contrivances of nature which we can- 

 not sufficiently admire." — Linn. Trans, xvi. 356. 



N 



