10 Prof. Mcintosh's Notes from the 



as a stout supporting layer external]}' and a central region 

 of complex reticulations. There is thus a considerable 

 divergence from the bone-forming periosteum or the peri- 

 chondrium of vertebrate cartilage, though the structureless 

 matrix of the latter with its enclosed cells comes nearest. The 

 great mass of this chordoid skeleton is dorsal, as are also the 

 ganglia, whilst the great nerve-cords rapidly seek a ventral 

 position, the former being above the alimentary canal, the 

 latter beneath it. The muscular fibres on the inner curve of the 

 chordoid skeleton about the level of the open vestibule — that 

 is, before the closure to form the oesophagus — are not longi- 

 tudinal, but oblique or vertical, stretching from the lower part 

 of the inner concavity to the upper part of the arch, so that 

 they would shorten the curve. Moreover, the "perichondria!''' 

 border shows large reticulations on its inner edge, a feature 

 of importance in the elasticity of the parts during the varied 

 movements (PI. II. fig. 10). The inner border of this tissue 

 widens at the level of the full development of the apparatus, 

 and at its broad lateral part the sides of the curve projecting 

 outward are laced together by muscular fibres, so that the 

 curve — acute as it is — can be shortened. At this level also 

 the chordoid central area is strengthened by special processes 

 of the marginal tissue (" perichondrial " of authors). At the 

 origin, again, of the chordoid skeleton (PI. III. fig. 15, ch.) 

 transverse muscles connect the two sides, and mesenterial 

 fibres pass from their lower edge to the oesophagus, whilst 

 the common duct of the thoracic glands is clasped by the 

 strands. It forms a protective shield and support to the 

 two great vascular trunks, the ccelomic spaces, and to 

 the cephalic ganglia, whilst stiffening the attachments of the 

 muscles of the region ; indeed, in extent, it exceeds the 

 cephalic skeleton of the cuttlefishes, and yet it has a certain 

 degree of elasticity in the varied and graceful movements 

 associated with the display of the branchiae. Passing forward 

 the lateral regions of this chordoid skeleton enlarge and 

 begin to present intruding pillars, cutting the outer edge 

 into regular spaces with convex margins externally, the first 

 indication of the bases of the branchial filaments. Then 

 the chordoid tissue arranges itself in long lobes connected 

 with a narrow and rapidly diminishing inner belt of the 

 same tissue, and this is soon followed by the disappearance 

 of the inner belt and the inner portion of each lobe, leaving 

 only a rounded or ovoid chordoid area marking the origin of 

 each filament (PI. II. fig. 12). The space occupied by the 

 chordoid arch is now the seat of a series of radially arranged 

 muscular bands, two for each filament, a connective-tissue 

 septum from each chordoid oval passing in transverse section 



