18 THE VOYAGE OF H.M.S. CHALLENGER. 



upper part of the stem is also distinguished from the more fully grown portion below it 

 by the presence of the " interarticular pores." These are small pore-like openings between 

 the successive joints which are situated in the re-entering angles between the interradial 

 ridges, and are therefore radial in position (PL XL ; PL XIII. fig. 7 ; PL XV. figs. 1,2; 

 PL XIX. figs. 2, 3 ; PL XXV. ; Pis. XXVIIL, XXIX., XXXIV.-XXXVIIL, XLIV., 

 XLVIII. ; PL XLIX. fig. 2 ; PL L. figs. 1, 3). They are produced by the apposition of 

 two faint grooves radiating outwards from the centres of the contiguous joints, which are 

 largest at their central ends and shallowest towards the periphery (PL XIII. fig. 9 ; PL 

 XXII. figs. 8, 10, 12 ; PL XXXVII. figs. 10-18, 20, 22). They do not reach the central 

 canal of the stem ; and so there is no communication eff"ected by the agency of these 

 pores between the internal vascular axis and the exterior. Similar openings occur in 

 many Comatulaj, leading into spaces between the upper surface of the centro-dorsal and 

 the under surfaces of the radials which rest upon it ;^ but they efi"ect no communication 

 between the body-cavity and the external medium. 



Until comparatively lately but little has been known respecting the termination of 

 the lower part of the Pentacrinus stem. Nearly all the specimens obtained had a 

 fractured stem, from which no conclusions could be drawn. From what we know of the 

 development of the Comatulaj we may fau-ly assume that the young Pentacrinus 

 commences life attached to some foreign body by means of a terminal plate (the dorso- 

 central) at the base of its stem, and a calcareous expansion subsequently developed around 

 it. Mature individuals have been found attached in this way to telegraph cables. 

 Capt. E. Cole of the telegraph steamer " Investigator " has reported to Prof. Agassiz 

 " that he has frequently brought up the West India telegraph cable on which Pentacrini 

 were attached, and that they are fixed, the basal extremity of the stem spreading slightly, 

 somewhat after the manner of Holopus, so that it requires considerable strength to 

 detach them."^ The condition of the lower part of the stem fragment of Pentacrinus 

 naresianus, represented in PL XXXa. fig. 4, is perhaps due to this mode of attachment. 



At the same time there appears to be ample evidence that a Pentacrinite may lead 

 the same sort of free life that a Comatula does, attaching itself temporarily by its cirri. 

 Sir Wyville Thomson long ago pointed out, in the ease of Pentacrimis decorus,^ "that the 

 animal seems to have had the power of detaching itself" at any of the syzygies of the 

 stem in the same sort of way as the arms are thrown off during life, or break up after 

 death. He described an individual in which the stem terminated below in a worn and 

 rounded nodal joint, and he supposed it " to have finally parted from its attachment and 

 to have led a free Life." He stated some years afterwards that this was the case in aU the 

 complete specimens which he had seen, " showing that the animal must have been for 

 long free from any attachment to the ground."* He then went on to describe the same 



■ The Genus Actinometra, loc. cit, pp. 8S-90, pi. viii. figs. 5, 7. 



- Bull. Mm. Comp. Zibl., vol. v. Ko. 14, p. 296. ^ Sea Lilies, p. T. 



' Tlie Depths of the Sea, pf. 442-44^1. 



