ME, P. H. CAEPENTEE OX THE GENUS ACTINOMETEA. 57 



as described by Simrotb 1 in Ophiactis virens, so as to form a network of delicate 

 threads without any imbedded nuclei, although it may contain pigment-granules ; and 

 this network passes very gradually into the nucleated protoplasmic plexus forming the 

 organic basis of the brachial segments (PI. III. fig. 7, L x ). 



The tendons of the stem of Pentacrinus are, I believe, of precisely the same character 

 as the ligamentous bundles between the arm-segments, although, of course, enormously 

 longer. They stain deeply with picro-carmine, and are composed of parallel fibres, which 

 may be teased out into very much finer ones, and their upper ends pass into the organic 

 ground-substance of the five basals, precisely in the same manner as the fibres of the 

 arm-ligaments pass into the protoplasmic network composing the organic basis of the 

 successive segments. 



In Pentacrinus Wyville-Thomsoni, in which the five basals are completely in contact 

 with one another in pairs, the two elements of every pair are united by a synostosis, 

 and the union of the basals with the radial pentagon above them is of the same cha- 

 racter. The first radials of Comatula are connected with one another and with the 

 centrodorsal piece in the same manner, as is seen in PI. III. figs. 5, 6, where L, I repre- 

 sent the tracts of fibrous tissue connecting the first radials with one another and with 

 the centrodorsal piece respectively. The terminal portions of this fibrous tissue become 

 calcified to form the compact superficial layers of calcareous substance on the apposed 

 faces, while the middle portion remains as the fibrous cement-substance uniting the two 

 calcareous segments, which is thus essentially of a connective-tissue nature. 



The mode of union of the segments of the calyx of the Tesselate Crinoids, none of 

 which are connected with one another by a muscular articulation like the first and 

 second radials of Pentacrinus and Comatula, was most probably a synostosis of the 

 same nature as those just described. The immovable sutural unions between certain of 

 the brachial segments to which Miiller gave the name of " syzygia," arc, in Pentacrinus, 

 of precisely the same nature as the synostoses between the segments of the calyx, the 

 organic basis of the one segment being continuous with that of the other through the 

 fibrous cement-substance, which forms a thin layer between the whole of the two simple 

 opposed surfaces. This was described by Miiller 2 as a very delicate membrane, of a 

 different nature from the elastic interarticular substance between the likewise sutu- 

 rally united stem-segments. 



In Comatula, however, the apposed surfaces of the two elements united by a syzygium 

 are not plain and simple, as in Pentacrinus and Bhizocrinus 3 , but marked by a series of 

 radiating ridges, as in Apiocrinus obconicus, Goldf. 4 The ridges of the two surfaces 

 correspond in position, and when the surfaces are in contact are closely applied to one 

 another, and united by fibrous cement-substance as in an ordinary synostosis. The fibrils 

 are very numerous and placed very close to one another, so that the calcareous reti- 

 culation forming the ridges is remarkably dense and compact, being formed around the 

 ends of these fibrils where they pass into the organic basis of the segments ; and these 

 ridges thus correspond to the whole of the syzygial surfaces in Pentacrinus and Rhizo- 



1 Op. cit. p. 435. - Eau des Pentacrinus, p. 20. 3 Sars, he. cit. p. 22. " Petref. German. Taf. lvii. fig. 5. 



