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



be aimed at is the understanding of things, and their description in words few and 

 familiar. Confusion does not arise from employing the same word in various ways 

 provided the context be well written. Does anybody fall into doubt about a yard, a 

 back-yard, a steel-yard, a yard-arm, a whin-yard, or a vine-yard ? A word changes 

 meaning with each new combination, or surrounding, or tone. No one mistakes the 

 sarcasm of, "You're a pretty fellow ! " or the tenderness of, " What a pretty child ! " 



DESCRIPTION OF SPECIES. 



Family, Ophiurid^. 



The Ophiuridse are a family in the order of Starfishes characterised by a more or less 

 sharply-defined central disk containing a simple digestive cavity which does not radiate 

 into the slender rounded arms, and has no anal opening. The arms have an axis composed 

 of jointed vertebra-like sections (arm bones), each made up of two ambulacral pieces 

 soldered side by side. The axis is cased wdth plates, of which the single row, covering 

 the under side, is peculiar. The plates on the sides bear spines. Each arm bone is pierced 

 by a water tulje, destitute of a bulb, and supplying the imperforate tentacle which is 

 bedded in the bone itself. The halves of the first two arm bones are swung laterally, into 

 the interbrachial space and soldered together to form the mouth angle, ^ and in them are 

 set the mouth tentacles which are watered by a forking tube from the mouth ring. On 

 either side of the base of each arm, above and below, run two stout pieces, the radial 

 shield and genital plate, which are joined at the margin of the disk and connected by an 

 adductor muscle. In the lower interbrachial space, parallel with and close to each arm, 

 are one or two genital openings that enter a peculiar sac, the genital bursa, with which 

 communicate spermatic or ovarial tubes. The inner angle of each lower interbrachial 

 space is occupied by a single plate, the mouth shield, and one of these serves as the 

 madreporic body.^ 



' Dr LuJwig considers the peristomial plate lying above the mouth angle as the junction of the first two ambulacral 

 pieces, a view I hesitate to adopt, since this plate is in no way coimected with either of the mouth tentacles, and because 

 it may be composed of one, two, or three pieces, or be wanting altogether. 



- For an epitome of the finer anatomy of Opliiurid», see P. H. Carpenter, the Minute Anatomy of Bracliiate 

 Echiuodenns, Quart. Joum. of Micros. Soc, April 1881, p. 169. 



For the bibliography of the two families, see T. Lyman, 111. Cat. Mus. Comp. Zool., No. i., 1865, p. 5; No. vi. p. 5, 

 1871. H. Ludwig, Echinodermen des Mittlemeeres, Mittheilungen des Zoologischen Station zu Ncapel, vol. L, No. 4, 

 1879. 



For description and comparative nomenclature of the hard parts see T. Lyman, Bull. Mus. Comp. Zool., vol. iii., part 

 10, 1874, p. 26U, pi. i. 



