176 On the Connexion between the {Sepr. 
to be worn away retain a partial continuity with the organic system 
of circulating fluids, as the organic bulbs of hair, the roots and 
lamelle of nails and hoofs ; whilst the other parts, which are des- 
tined to be shed, as feathers and cuticular scales, are wholly de- — 
tached from the vascular communion after their complete forma- 
tion, and only adhere mechanically to the living parts for a time. 
The most apposite illustrations, and the most positive instances, 
of union between vital and extra vital parts are to be found in the 
testaceous tribe of animals. After a long continued and careful in- 
vestigation, } am fully convinced that the shells of all the vermes of 
Linnzus are extra vascular from their commencement, and remain 
so during their whole connexion with the living creature. The first 
production and the growth of those shells always depends upon a 
deposit of the material thrown out from the surface of the body 
of the living animal. The figure and colours of the several parts 
of those shells in every species depend upon the shape and the 
colouring glands of the modelling organs: fractures are repaired by 
spreading a cretaceous fluid over the inner edges, and never by any 
exudation from the fractured parts, since they always retain the 
angular broken surfaces after such repairs. Extraneous bodies are 
equally coated with shell, whether they are in contact with the 
parent shell or not. The first may be seen in the frequent envelope- 
ment of Neides in the common oyster ; and the latter has been often 
ascertained by experiments made for the purpose of creating artifi- 
cial pearls, and which might, if skilfully practised, yet prove very 
successful. The borings of parasitical vermes into shells are never 
filled up, or the bored surfaces altered, unless such borings pene- 
trate into the cavity where the living animal dwells, and then the 
apertures are invariably plugged up, or smeared over with pearly 
matter. ‘The water-worn outer surfaces of old shells, and other ex- 
ternal abrasions, are never repaired, which is to be seen in old 
living oysters exposed to the moving friction of currents or strong 
tides ; in the worn-off spines of the pholas dactylis; and in the 
convex points of the two valves of old mzyfzi, especially the mytilis 
anatinus. I have sought in the most extensive collections of the 
metropolis for examples of fractures, and other injuries, which have 
occurred to the shells of living vermes, and J have collected many 
remarkable specimens. They all demonstrate the same results, 
without any exception. I have made numerous experiments upow 
the garden snail (helix nemoralis), by fracturing and breaking away 
the shell in various parts; and have always found the repairs to be 
effected from within by first smearing over an epidermoid varnish, 
and then by plastering the inner surface of that fitm with successive 
calcareous lamine. | have in vain attempted to inject the shells of 
recent vermes from the vascular parts of their bodies, and am fully 
satisfied that none of their albuminous or gelatinous testaceous mem- 
branes were ever at any time permeable to vessels; indeed they do 
Not possess any of the reticular texture or arborescent pores which 
are common to all vascular parts ; but, microscopically examined; 
ce 
