BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 377 



distinctions — based on external characters, and the form of the nerve-ganglia — 

 given by Prof. Keferstein, its independent position can be more satisfactorily 

 demonstrated. Externally (Plate X. fig. 3) there is the usual ciliated coating, 

 whose action is most vigorous in the cephalic region. The cutaneous textures are 

 exceedingly transparent, the pigment, if present, being only developed at the 

 snout in front of the ganglia as a rose-pink or reddish shading within the super- 

 ficial cuticular layer of the parts. The cutis (d), composed of the usual granular 

 cells and gelatinous matter in areolee, has along its inner margin a trace of a trans- 

 lucent homogeneous basement-layer. A very thin layer of circular fibres (/) comes 

 next, the exact structure of which is best demonstrated in the fresh animals, after 

 the addition of a little dilute acetic acid. The fibres are also evident in fine longi- 

 tudinal sections, but are not satisfactorily seen in transverse sections on account 

 of their tenuity. Beneath this lies a very powerful longitudinal muscular coat 

 (e"), the cut ends of the fibres having the usual fasciculated appearance, the inner 

 being somewhat coarser than the outer. At each side a distinct increase occurs at 

 the region of the nerve, where the coat is separated into two portions by a septum 

 of fibres from the circular coat, the nerve lying in the line of demarcation. This 

 arrangement is quite characteristic, and the position of the nerve-trunk probably 

 points to the compound nature of the great longitudinal layer, viz., as analogous 

 to the two longitudinal layers in Borlasia, the circular muscular coat cutting off 

 only the lateral portions (e), instead of dividing it completely. This genus shows 

 the mobility of the race even in a greater degree than the others. In crawling 

 about the long yielding snout is used as an exploratory or boring organ, which it 

 stretches hither and thither with ceaseless energy, and by its aid is able to push 

 aside its own mobile body in any direction ; while through any narrow loop of 

 mucus the latter is drawn like a thread of semi-fluid, yet coherent substance. 

 These animals also progress readily on the surface of the water. When tested with 

 blue litmus-paper the skin of Cephalothrix gives a most vivid red stain. 



Delle Chiaje's* description of the structure of the body-wall, if applied to the 

 Ommatopleans, is correct enough, viz., that there is an external layer of circular 

 fibres and an internal longitudinal coat ; hence the criticism of M. de Quatrefages 

 requires qualification. The Polia siphunculus, D. Gh., however, seems to have 

 been a Borlasian, judging from the large triangular slit which lies at a consider- 

 able distance behind the snout. H. RathkeI gives Borlasia striata two coats, — an 

 epidermis, and a corium, — combining under the latter both the pale and the pig- 

 mentary layers of the skin. He has omitted to notice the external longitudinal 

 muscular layer, and mentions only an outer circular and an inner longitudinal 

 muscular coat. It is somewhat difficult to comprehend the views held by M. de 

 Quatrefages with regard to the same structures, since his descriptions and 



* Memorie sulla storia, &c, vol. ii. ] 825. 



f Neueste Schriften der Naturforschenden, &c. p. 95, 1842. 



VOL. XXV. PART II. 5 I> 



