ME. P. H. CAEPENTEE ON THE GEXUS ACTINOMETEA. 55 



mass which occupies the whole space between the opposed faces on the dorsal side of 

 this ridge, describing them as alike consisting of elastic interarticular substance, tlie 

 function of which is extensor and antagonistic to the flexor action of the muscles. 

 Dr. Carpenter ', however, regards the former as interarticular, with the special function 

 of holding the pieces together, but allowing a certain amount of movement between 

 them, Avhile he describes the single dorsal mass as elastic, and as antagonizing by its 

 extensile powers the action of the flexor muscles. Histologically he finds no differ- 

 ence between them, both consisting of minute, straight, and nearly parallel fibres, very 

 much, in fact, like those which Miiller described as composing the tendons of the stem of 

 JPentucrimts. At their points of attachment to the pieces of the skeleton these fibres 

 pass into their basis substance and become incorporated with it. 



The union of the first radials with one another and with the centrodorsal piece, which 

 had been spoken of as sutural by Miiller, is described by Dr. Carpenter as "an 

 adhesion of expanded surfaces closely fitted together, and held together by the con- 

 tinuity of their sarcodic basis substance " Q). 704) ; so that the different elements are 

 cemented together by a " thin layer of sarcodic substance, continuous with that which 

 occupies the meshwork of their own calcareous reticulation." This mode of union may 

 be conveniently described by the word " synostosis," which has been employed by 

 Simroth to designate the mode of union of two faces which " verkitten sich " in the 

 skeleton of the Oplduridce. It is essentially the same as the syzygial uniou which 

 occurs between certain pairs of the primitive arm-segments, although differing from it in 

 points of detail. 



(§ 36) We have seen that Miiller regarded the tendinous tissue of the stem of Penta- 

 criiius, and the fibrous ligamentous bundles, or, as he called it, the elastic interarticular 

 substance uniting the movable elements of the skeleton both of JPentacrinus and of 

 Comatula, as distinct from one another. I believe, however, that they are fundamentally 

 identical, not only with one another, but also with the so-called " cement-substance " 

 between two segments which are united by synostosis. This last consists, in the Oj)hin- 

 ridce, according to Simroth ", of connective-tissue fibres which lose themselves in tlie 

 organic basis of tlie skeleton, and are of the same nature as the substance of the 

 masses of connective tissue uniting two articulating surfaces, both tissues staining 

 deeply with picro-carmine. I find the same to be the case in Comatula and Penlacrinus. 

 The tendons of the stem of the latter genus, the ligamentous bundles, composed, according 

 to Miiller, of elastic interarticular substance, which connect every pair of movable 

 arm-segments, and, lastly, the "cement-substance," uniting the first I'adials to one 

 another and to the centrodorsal piece, all stain very deeply with picro-carmine, and are 

 of essentiallj^ the same histological structure. 



Pig. 4, on Plate III., represents a portion of a horizontal section through the suture, 

 or, as I prefer to call it, the synostosis of two of the first radials of I'entacriuus. In the 

 immediate neighbourhood of their apposed lateral faces there are none of the nuclei nor 

 pigment-granules which are imbedded so abundantly in the more internal portions of their 

 protoplasmic ground-substance, and the threads of the plexus of which it is composed 

 ' Phil. Trans, he. cit. pp. 703-714. ' 0^. clt. p. 43-5. 



