48 Prof. M'Intosh's Notes from the 



and may be present or absent, as in Filograna, with per- 

 plexing indifference, whilst in other forms their stability 

 and characteristic shape have made them of specific im- 

 portance. It is interesting in connection with ' the 

 branchial view of the opercular stalk that transverse bars 

 of bluish pigment are occasionally seen on it. 



Muscular System. — Immediately behind the brain muscular 

 bauds pass from the sides of the ventral to the dorsal wall 

 (or vice versd), some of the same side being attached to the 

 base of the opercular stalk dorsally — indeed, they seem to be 

 strongest and best developed at first on that side. Ventrally 

 they are inserted on each side of the nerve-cord, and 

 by-and-by they bound the thoracic glandular organ on its 

 inner border. 



Behind the ganglia and the opercular stalk the body-wall 

 assumes a more symmetrical outline, and the dorsal longi- 

 tudinal muscles become more distinct and quite separate 

 from each other, but the ventral longitudinal muscles are 

 indistinguishable. In the median ventral region, however, a 

 special thin longitudinal muscular band occurs on each side, 

 and continues backward a short distance — disappearing as 

 the actual ventral longitudinal muscles become distinct. 

 These ventral longitudinal muscles are formed by fibres on 

 the lateral region of the body-wall outside the anterior 

 glandular organ and its appendix, and not in contact with 

 the nerve-cords, which are separated from them by a con- 

 siderable interval. Their outline in transverse section is 

 elliptical, and, as the glandular organ in its progress back- 

 ward diminishes, the fibres seem to pass externally ; then, as 

 the glandular tube disappears they form a thin stratum to 

 the outer side of the nerve-trunks and in contact with 

 them, the anterior median ventral fibres being still visible 

 between the nerve- trunks. By-and-by the median, or pseudo- 

 ventral, or anterior ventral, fibres (PI. V. fig. 26, mr) dis- 

 appear from the middle line, and the ventral longitudinal 

 form a spindle-shaped" layer in section, separated by an 

 interval from the dorsal, which bend inward at their lower 

 ends, whereas the ventral pass outward below and beyond 

 them. The dorsal and the ventral longitudinal muscles, 

 however, by-and-by fall into line and the body-wall becomes 

 more compact, the dorsal muscles retaining the great 

 preponderance in bulk, and closely approximated to the 

 ventral, only a slight incurvation of the inner surface and 

 traces of the oblique muscle indicating the line of separation ; 

 vet the distinctly pennate arrangement of the fasciculi 

 of the dorsal is characteristic. The nerve-cords are more 



