132 MARINE ANIMALS. 



but this applies only to the nacre, or mother of pearl, for the same appear- 

 ance is not perceived in the subnacreous formation, which is common in most 

 of these shells, and is in fact only a modification of the 

 perfect structure. From the most beautiful shells the 

 most valuable pearls are extracted, of the structure of 

 which I shall speak in another place. In some of the 

 Cockles, and to a certain extent in all, the hinge presents 

 a series of teeth, which fit into one another in the interstices 

 . „„ , . between each with beautiful accuracy, and in the Mutilus 



Fio. 7.— The mode m _ •> ^ 



■which the membrane is Triqonia the ccUular formation is simply spherical, and not 



folded to ffive the colour "^ . _ c j r j 



to the mother of pearl, hexagonal, as in the Pinna tribe. In some species a partial 

 absorbence of the cells is observable, and this, I believe, takes place to a 

 certain extent in all. 



The parasitical habits of Marino Animals are by no means confined to the 

 Barnacles and the like species, for in the Conchifera we find some very 

 curious and not unpicturesque examples. Thus there is a species of Pinna 

 known as the ^^Tree Oyster," which fixes itself upon some Gorgonia or other 

 Vesiculosus, and has the appearance of a branching plant with shell-like leaves 

 or flowers at the extremities of the branches. On examining these they 

 appear transversely ridged, and exhibit the true structure of the Pinna 

 tribe, only that, for the purpose of adhering to the plant upon which they 

 fix, they have a series of clasping ends to the ridges not unlike claws, and 

 soine of the sm iller ones have exceedingly the appearance of caterpillars; 

 they are, when full grown, of a pear-like shape, and have a depression within 

 the upper edge. One very remarkable circumstance connected with their 

 growth is, that they retain exactly the same size where they adhere, and no 

 lamina are added on the under surface, but all on the upper, so that the 

 plant or sea-weed upon which they grow is not in the least injured by any 

 pressure. Animils of all kinds make their habitation on the surface of the 

 shells of Conchiferous auim.ils, and interesting instances have occurred in which, 

 as though for concealment, shells of a smooth surface attached themselves to 

 those which were serrated or ridged, and took the same form. Barnacles, 

 Sea-worms, and hundreds of minute creatures not only reside upon, but bore 

 holes into the.se shells, and Corallines and Sponges adhere to them; so that 

 it is not only in civilizad life, or the higher order of beings, that the lower 

 are dependent on the higher. 



(To be continued.) 



