220 A MONOGRAPH OF THE 



of these narrow tubes near its base, one or two spicula may 

 be seen, but frequently for a considerable distance from the 

 last spiculuin the sponge consists of sarcode only, and in no 

 case were any of Mr. Hancock's supposed erosive organs to 

 be detected in conjunction with the sponge. 



The inner surfaces of these cells have a peculiar character ; 

 they are not regularly and evenly bored, but the parietes 

 consist of numerous small concavities, which appear to have 

 been each bored separately, so that the margin of the last 

 one partially obliterates those of its neighbours, and their 

 average diameter is about that of one of the leading canals ; 

 thus it would appear that the same excavating apparatus 

 that applied in a continuously straight direction that formed 

 a tube, was applied in a different manner to enlarge the 

 terminal portion of the cavity by a series of short, lateral 

 borings, and thus to form a large cavity, into which the An- 

 nelid producing it might retire at its leisure, or making it 

 its head- quarters, bore thence other tubes in various direc- 

 tions, to suit its own habits and purposes, and when these 

 were served, and it had either retired from them or died, 

 the excavations were taken possession of by the sponge, 

 just as a Pagurus takes possession of a dead univalve shell. 

 The Annelids found in the borings in the Balani at Sark, 

 and those in the deeply-seated borings of the limestone 

 rocks at Tenby, were distinctly different species ; the former 

 being probably lithophagous, or perhaps more correctly, 

 testophagous in their habits, while the former were litho- 

 domus only. 



It is well-known to naturalists that the common earth- 

 worm passes the vegetable mould through its intestines, 

 separating the nutrient matter incorporated with it by di- 

 gestion, and ejecting the indigestible matter ; the powerful 

 jaws of the Annelids inhabiting the various species of shells 

 eroding their substance with o;reat facilitv. The substance 

 thus detached, abounding in animal matter, is most pro- 

 bably passed through their digestive organs, thus affording 

 nutriment, and forming a safe habitation at the same time, 

 and accounting also for the vast number of perforated shells 

 and the comparative rarity of the annelids. 



