472 



ANNELIDA. 



of Vermetus, whilst the latter are partitioned by shelly septa, which 

 do not exist in the former. Again, the tube of Dentalium is open 

 at both ends, whereas it is closed at one extremity in the Serpula. 

 In the Annelidous genus Ditrupa, however, the tube is open at 

 both ends, so that this distinction is one not universally applicable. 

 Tubicolar Annelides are certainly known from the Ordovician 

 rocks upwards, almost every great period having representatives of 

 the order, though many of the fossils referred to this group are of 

 a more or less problematical nature. The genus Spirorbis has 

 survived from the Silurian period to the present day; and forms 

 apparently not separable from the existing genus Serpula are known 

 in deposits at any rate as old as the Carboniferous. 



Of the fossil Tubicolar Annelides which are likewise known by 

 still existing types, and the true relations of which are therefore 

 beyond dispute, the three chief genera are Serpula (including Ver- 

 milia and Filogrand), Spirorbis, and Ditrupa. In the genus Serpula 



(fig. 334), there is a long shelly 

 tube, usually more or less tortu- 

 ous, sometimes solitary, some- 

 times aggregated, which is fixed 

 to some foreign body by a smaller 

 or larger portion of its surface. 

 The genera Vermilia and Filo- 

 grana are principally distinguish- 

 able from Serpula proper by 

 characters belonging to the ani- 

 mal and not to the tube which 

 it inhabits. It is doubtful, there- 

 fore, if these three types can be 

 separated in the fossil condition 

 from one another ; though it has 

 been adduced as a distinctive 

 character of Vermilia that the 

 mouth of the tube often pos- 

 sesses from one to three tooth- 

 like projections. 



As regards its microscopic 

 structure, the tube of Serpula 

 appears to be mainly composed 

 of delicate concentric laminae of carbonate of lime, with a consider- 

 able proportion of animal matter. Some of the layers, and par- 

 ticularly those near the exterior, may be of considerable thickness, 

 and may exhibit a peculiar tubulated structure, being traversed by 

 minute branching canals, the direction of which is at right angles to 

 the long axis of the tube itself (fig. 335, a). In other cases, the 



Fig. 334.— Serpula JZagelZuin. 

 (Jurassic.) 



Oxford Clay. 



