372 DR W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



greater development of pigment. The body is everywhere covered with cilia, 

 which are most active in the lateral Assures, but longest on the papillae of 

 the snout. They may be seen in active motion under a lens in good light. 

 Sometimes the motion of the cilia in the lateral fissures is suspended, and again 

 set agoing, without evident cause. Dr Williams first asserts that the cilia are 

 confined to the dorsal half of the body,* and then seems to contradict himself by 

 saying farther on that the whole body is ciliated.f The latter, as above-men- 

 tioned, is the correct view. 



In the living animal the cutis has a cellular aspect (Plate IX. fig. 4), the cells 

 or areolae measuring 10 1 0o th of an inch or more, and most distinctly seen towards 

 the tip of the tail in the adult. Sometimes a number of minute clear granules are 

 observed overlying the larger cells, as shown at the lower third of the figure. The 

 pigment-cells and granules reach their greatest development anteriorly, and some 

 of the former contain very dark brownish black pigment in circumscribed masses. 

 The dorsal pigment has in general a longitudinally streaked appearance (Plate 

 IX. fig. 5), a state probably due to the peculiar arrangement of the fibres of the 

 external muscular layer hereafter to be described. In some pale red specimens 

 the coloration is observed to be due to a uniform impregnation of the cutis, and 

 the tint is much deeper than that of the ganglia, which are thus rendered 

 conspicuous by their pallor. Occasionally one or two pigment-cells of exception- 

 ally large size are present anteriorly (Plate IX. fig. 6), and there were three clear 

 granules in the larger of the two figured. The cuticular cells are finer in Borlasia 

 lactea, Mont. MS., and the body is not clouded by the granular pigmentary 

 matter. The superficial arrangement in Meckelia annulata (Plate IX. fig. 7) is 

 similar, though the cells or areolae are smaller, and the pigment-granules do not 

 form themselves into streaks. 



There are three tactile papillae on the snout, one of which, from its situation, 

 falls to be described with the opening of the canal for the proboscis. The other 

 two are placed on each side of the central (Plate X. fig. 1), but are not always so 

 prominent. Each is furnished with a series of cilia of greater length than those 

 on the general surface, and which extend from the erected papilla in a radiating 

 or fan-shaped manner. They are probably of great tactile service to the worm. 

 Prof. Keferstein refers to a "transverse" tactile papilla on the snout of his 

 Cephalothrix longissima, which differs from those usually seen in Borlasia, and 

 resembles a slight pouting of the lining membrane of the canal for the proboscis. 



Under pressure granular masses and globules of mucus resembling oil are ex- 

 truded from the skin, as in Ommatoplea, and often congregate round the borders of 

 fresh transverse sections. But, while in Ommatoplea there are only the ciliated and 

 structureless epidermis, a single layer of cutis-cells and the basement-layer, before 



* Report Brit. Assoc. 1851, p. 171. f °1 J - cit P- 243 « 



