5G MK. P. H. CAEPENTER ON THE GEXUS ACTINO-METRA. 



become excessively attenuated and disposed with great regularity almost parallel to one 

 another. At the same time the meshes of this organic plexus become greatly elongated 

 in the intervals between the parallel thi-eads or iibres, which are connected with one 

 another by very delicate fibrils passing in the form of loops from one fibre to another. 

 These loops, which forcibly recall Miiller's description of the arcades connecting the 

 fibrous columns of the elastic interarticular substance in the stem of Pentacrimis, are 

 simply the expression of the ends of elongated meshes of the protoplasmic plexus 

 forming the organic basis of the skeleton. In the neighbourhood of each of the two 

 opposed siu'faces the fibrous elements of this plexus assume the character of closely 

 placed parallel connective-tissue fibres, Avith no pigment-granules nor nuclei imbed- 

 ded in them, but staining deeply Avith picro-carmine, Avhile the normal protoplasmic 

 basis of the interior part of the calcareous segments is but little affected by this 

 reagent. These fibres pass from the organic basis of the one segment into that of the 

 other so that the two are firmly united, and the superficial denser layer of calcareous 

 tissue is deposited around their ends, which corresponds with ^liiller's descrii:)tion of the 

 elastic interarticular substance of the stem of Pentncrlmts as extending for a short 

 distance into the calcareous substance of the opposed faces of the segments. The 

 superficial layer of calcareous reticulation Avhich occupies the small intervals between 

 the ends of the fibres thus becomes extremely close and compact ; but the central 

 portion of the fibrous tissue (PI. III. fig. 4, L) does not calcify, remaining as a thin layer 

 of fibrous cement-substance betAveen the two opposed surfaces, precisely like the inter- 

 articular substance in the stem o£ PeiitacriuKif, Avith Avhich I believe it to be identical. 

 It is, at any rate, of the same nature as the su1)stance of tlie ligaments connecting the 

 first and second radials, Avhich Miiller described as identical Avith that connecting the 

 stem-segments ; for at the angles of the radial pentagon the fibres of the cement-sub- 

 stance connecting the adjacent first radials with one another in pairs pass directly into 

 the fibres of the ligamentous bundles between the first and second radials. These, Avhich 

 are of precisely the same character as tlic ligamentous bundles betAveen the successi\'e 

 brachial segments, also stain deeply Avith picro-carmine, and only dilfer from the cement- 

 substance in the greater length of their fibrous element. 



At the points of attachment of the ligaments to the pieces of the skeleton, the meshes 

 of the organic plexus become greatly elongated, and its fibrous bars regularly disposed 

 and connected Avith one another by loops, as above described. As, howcA-cr, the distance 

 between the two articulating faces is very much greater than in a synostosis, several of 

 these minute primitive fil)rils unite to form one of tlie larger fibres composing the liga- 

 mentous bundle, at the other attachment of which these primitive iibres again separate, 

 become connected Avith one another by transverse loops, and finally pass into the bars 

 of the protoplasmic plexus forming the ground-substance of the next segment. 



(§ 37) I have found the fibres composing the ligamentous bundles between the 

 arm-segments of Antedoii to terminate in the manner above described for Pentacrimis; 

 but in Act. polijmorpha they do not pass so directly into the organic basis of the seg- 

 ment. At the ends of the ligamentous bundles, where their component fibres begin to 

 break up into primitive fibi'ils, the latter cross one another in all directions, very much 



