4:50 Piof. M'lntosh on the 



the median septum. The histological characters of tlie lower 

 half just described differ from the superior, in so far as the 

 whole section is more granular and the strife less distinct ; 

 indeed, in most views it presents a glandular appearance 

 with numerous concentrically arranged strise. Soft cellular 

 substance generally fills up the space in the centre. When 

 the crown-like appeai'ance in section has attained full 

 development, the muscle in each central space is most 

 bulky internally, and the fasciculi from the median septum 

 extend further downward, the arrangement having a some- 

 what ])ennate appearance. Before the separation of these 

 chambers occurs (i. e. when they have assumed the form of a 

 figure of eiglit) the muscle occupies about three fovirths of 

 the diminished area, passing from the upper half of the short 

 median septum, and extending past the junction of the 

 external chamber superiorly. The direction of the muscular 

 fasciculi is also more horizontal — except at the outer or 

 superior border, — and the cellular or glandular tissue is much 

 less. The latter becomes still more diminished, and the 

 fasciculi in the nearly circular area are directed downward 

 and inwaid, the most conspicuous occupying the middle. 

 Finally the muscle ceases in ordinary sections in a line with 

 the tentacles. The median muscles are thus confined 

 entirely to the snout, and, from their relations with the 

 chitinous environment, act in various ways on the flattened 

 organ. The presence of the cellular or glandular tissue 

 within the same sheath and its changes in those bearing the 

 lateral organs are also noteworthy. The whole tissues of the 

 snout, indeed, sympathize with such changes, for the muscles 

 become less firm and more granular, the chitinous supports 

 less distinct, and the blood-vessels, as will be seen afterwards, 

 shrink to short trunks about a third their ordinary length. 



The lateral muscles of the snout take origin anteriorly a 

 little behind the former, gradually widen into broad ribbon- 

 shaped bands, then become narrower toward the mouth, and 

 are continued backward as the ventral longitudinal muscles 

 of the body-wall. Their fibres are chieHy longitudinal in 

 direction, and in partial contraction the muscles in the snout 

 assume a barred appearance at the outer margins — the bars 

 or ridges slanting from without inward and slightly back- 

 ward. Careful examination in the fresh specimen also 

 reveals a series of very fine transverse fibres, which, from 

 the triangular region in front of the mouth to the anterior 

 third, slant forward and outward. In general arrangement 

 these muscles present in the snout certain well-marked 

 differences from their subsequent structure. In front each 



