51 ME. P. II. CAKPENTEll ON THE GENUS ACTINOMETRA. 



length of the stem, passing tlirough the substance of its various segments, and (2) l)y the 

 " ekastie iutcrarticular substance" between the individual se2:mcnts. 



The substance of these tendons consists of a Avhite fibrillar tissue very like the ten- 

 dinous tissue of the higher animals ; but Miillcr supposed the elastic iutcrarticular sub- 

 stance to be of a totally different nature, consisting of " lautcr senkrecht stehenden 

 Fascrsiiulchen, die durch Rcihen bogenformigcr Schlingen einfachor Fasern verbunden 

 sind," and " diese Schlingen gohcn mit den rcgelmiissigsten Arkadon in ganz gleichcn. 

 Abstiindeu aus einem Fasersiiulcheu in das andere iiber." This substance fills up 

 the whole space between the successive stem-segments which is not occupied by the 

 tendons, and is connected in the closest possible manner with the opposed surfaces of 

 every pair of segments, even extending for a short distance into their superficial cal- 

 careous tissue. Each of the arcades above mentioned consists of a single primitive 

 fibre, the terminations of which arc lost in the " Fascrsiiulchen ;" and the passage of 

 these fibres in loops from one fibrous column to another gives an elasticity to the whole 

 tissue, viz. a power of contraction after lateral displacement, and of extension after 

 vertical compression, although the individual fibres are not elastic in the ordinary 

 sense of the word. 



Miiller described the basals of Fcntacrinus as united with the top stem-segments in 

 the same manner as tlie successive stem-segments with one another, namely, by this 

 clastic iutcrarticular substance, while their sides simply "stossen an cinander " (p. 25). 

 This mode of union between the stem-segments was generally called by him a "Xath," 

 or suture; but he sometimes spoke of it as an articulation, though he usually employed 

 this last term only in tliose cases in which two segments are movable on one another 

 through the intervention of muscles and ligaments which pass between thera. 



He further described the union between the first radials and the basals of Pentacrinus 

 and between the first radials and the ccntrodorsal piece of Comalula as a suture, which 

 name he also gave to the lateral union of tlic five first radials Avith one another (pp. 28, 29); 

 but he does not seem to have supposed that in these cases the various elements were 

 connected by the elastic iutcrarticular substance which he found between the likewise 

 suturally united stem-segments. In fact, in speaking of the syzygia, Avhich he called an 

 immovable sutural union of two segments, he said expressly that not only the muscles 

 but also the clastic iutcrarticular substance was absent. On the other hand, the latter is 

 to be found between the segments which are capable of motion vipon one another, whether 

 ligaments and muscles be present, as between the first and second radials and between 

 most of the brachials, or ligaments only, as between the first and second brachials and the 

 second and third radials ; for Miiller described the ligaments connecting two mutually 

 movable seirments as havinur essentiallv the same structure as the clastic iutcrarticular 

 substance of the stem, except that their surface is plain and not " krausenartig gefaltct" 

 (pp. 30-38). 



In those more common cases in which there is a muscular union between two segments, 

 such as the first and second radials, in contact by transverse articular ridges upon their 

 opposed faces (PI. VII. figs. 1 i, 2 a, 4 b,ba; i), Miiller drew no distinction between the 

 pair of ligamentous bundles on the ventral side of the articular ridge, and the single 



