13 



be drawn between those of the ectoderm and those of the 

 cutis, except in young specimens. Where the subjacent 

 tissue is calcified, especially along the dorsal and dorso- 

 lateral surfaces of the apical plates and the brachial 

 segments, they become fusiform and still more regularly 

 arranged, and the delicate filamentous processes into 

 which their basal ends are divided are continuous with 

 similar processes of many of the cells of the deeper layer 

 (PI. IL, fig. 26). 



Muscles and Ligaments. 



The flexor muscles of the arms and pinnules (PI. Y., 

 figs. 50 and 52, fix. m.) consist of large bundles of parallel 

 fibres which liave a strong affinity for aniline stains, 

 especially heematojcylin. Each fibre has a minute oval 

 nucleus, and is of uniform diameter throughout its length 

 (PI. III., fig. 36), but it is thicker along one edge than the 

 other, and thus presents a wedge-shaped figure in trans- 

 verse section. The ends of these fibres are sharply defined. 

 The extensor fibres are of extreme fineness and have less 

 affinity for stains (PI. III., fig. 35; PI. V., fig. 50). In 

 longitudinal sections they frequently possess a more or 

 less wav}^ appearance. Their ends are split up into a 

 variable number of fibrillsp, each of which may be traced 

 into continuity with the cells of the stroma which forms 

 the organic basis of the adjoining ossicles. The nuclei 

 are smaller and rather more rounded than those of the 

 flexor fibres. The fibres which form the interarticular 

 ligaments are not distinguishable from the extensor fibres, 

 and the interarticular fibres of the cirri are of similar 

 character. 



