BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 373 



the circular (external) muscular fibres are reached, in Borlasia the structure of the 

 dermal layer is more complicated. Fine tranverse sections of B. olivacea demon- 

 strate that underneath the ciliated epidermis (c, Plate XL fig. 8), a somewhat 

 thick layer (d) composed of granular cells and globules in areolae, occurs. From 

 the facility with which these contents escape, the drawings show the parts in a 

 slightly altered condition. Beneath this lies a pale structureless basement-layer 

 (df), the presence of which in Cerebratulus had misled Prof. Keferstein into the 

 idea that it was a layer of circular muscular fibres ; but an attentive examination 

 of that genus, as well as the present, demonstrates that, while one may be 

 deceived if only transverse sections are made, no doubt can exist in longitudinal 

 sections. This point may readily be settled without reference to the more 

 explicit, because larger, condition of the parts in the great Linens longissimus. 

 A thick compound layer is next encountered in B. olivacea, consisting externally 

 of pigment-granules and cuticular globules (d"), and internally of a series of 

 powerful longitudinal muscular fibres (e). Under a low power, indeed, this com- 

 pound layer in transverse section appears as one, the pigment and other cells, 

 and the cut ends of the muscular fibres, presenting a similar aspect. The amount 

 of pigment varies of course in different specimens, and is always much more 

 developed dorsally than ventrally. Towards the anterior end of the animal this 

 layer of the cutis {d") becomes thicker, and its reticulations more distinctly 

 marked. Fine longitudinal sections of the snout from above downwards show 

 superficially a series of very beautiful reticulations of a somewhat regular aspect, 

 the chief interstitial bands having a longitudinal direction. Towards the tip of 

 the snout the texture becomes denser in transverse section (Plate X. fig. 4), and 

 the pigmentary matter increases, especially just within the pale external layer of 

 the cutis. A section still further back (Plate XII. fig. 2) exhibits a less dense 

 arrangement, and the pigment is now for the most part grouped into a dorsal and 

 ventral band. The general stroma consists of radiating and longitudinal fibres, 

 the cut ends and granular matter being often situated in the axils of the radiating 

 series. The pigment anteriorly attains its greatest density immediately beneath 

 the pale external layer of the cutis, diminishing in quantity from this point 

 inwards. The snouts of these mobile animals resemble in structure the elaborate 

 arrangements which are sometimes met with in certain organs (such as the 

 tongue) in the higher animals, where extensive and delicate motions are combined 

 with great tactile power. 



In Cerebratulus bilineatus* the arrangement of the two white median dorsal 

 stripes is characteristic, for the pigment is strictly confined to the region cor- 

 responding to d" and e in Borlasia ; and in transverse section they appear as 

 two patches with an intervening pale space, bounded anteriorly by the basement- 



* Gordius tcenia, Dalyell, Pow. Creat. vol. ii. 

 VOL. XXV. PART II. 5 C 



