122 



THE VOYAGE OF H.M.S. CHALLENGER. 



axial cords run in tlie partitions between them. These branches have no regular mode of 

 subdivision, no two pinnules being exactly alike ; while they arc not symmetrical on the 

 two sides of the same pinnule. Longitudinal sections of the pinnules of Actinometra 

 nigra ' show that these branches which come up to the ventral side in successive segments 

 of the pinnules (woodcuts, figs. 4-7, a') are united by continuous trunks that run along 

 the upper surface of the pinnule right and left of the ambulacrum (woodcut, fig. 7, It); 

 they send branches upwards between the connective tissue spaces, of the same kind as 

 those which appear in transverse section.^ The nervous structures in these pinnules thus 

 consist of four principal trunks, three of which are intimately united Ijy a network of 

 fibrils, while the fourth and smallest is the ambulacral nerve. This is not yet known 

 to be connected with any other structures, though I strongly suspect that it is continuous 

 at the sides of the food-groove with a subepidermic plexus covering the pinnule and 

 communicating with the numerous branches of the axial cord. 



Fig. 7. — Longitudinal section of the ventral peiisome in a pinnule of Actinometra nirjra, x 60. 

 a'. Ventral ascending branch of the axial cord ; c.s, connective tissue spaces in the perisome ; ct, connective tissue above 



the ovary ; e, epidermis ; Ic, lateral canal, connecting the cceliac and subtentacular canals ; It, one of the lateral trunks 



which connects the ascending branches of the axial cord (a'). 

 N.B. — This section passes to one side of the medio-ventral line of the pinnule. 



Sections of the arms and pinnules of Antedon eschrichti give much the same results. 

 The ventral branches of the axial cords in the arms extend upwards along the sides of 

 the coeliac canal, curve round the outer walls of the subtentacular canal, and pass on into 

 the elevated folds of tissue bounding the food-groove, as shown in PI. LX. fig. 6, a'. 

 They do not seem to subdivide so freely as in the tropical Actinometra, but in both 

 genera I have traced their smaller fibrils into the little respiratory leaflets along the edges 

 of the food-groove. Pcrrier has seen the same thing in Antedon rosacea, and believes 



1 Theel's figure of a dorsal nerve-trunk in Elpidia glacialis with its muscular branches {K. Svensk. Vetemk. Akad_ 

 Handi, Bd. 14, No. 8, Taf. iv. fig. 18) has a wonderful resemblance to a longitudinal section of the axial cord in a 

 Crinoid pinnule. 



2 Woodcuts figs. 4 and 5 are composite figures made up from tlie stud}' of some half dozen sections through the 

 central part of a pinnule-joint and tlie overlying ventral perisome. Woodcuts figs. 6 and 7, however, are diagrammatic 

 representations of single sections, and I have plenty more of the same character. 



