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



discriminated as a "skeleton" in Cephalodiscus by Professor Ray Lankester, for it had 

 only been indicated as a septum in my preliminary account in the Annals of Natural 

 History. The examination of fresh specimens of Rhabdopleura off the Norwegian 

 coast had enabled him to detect the existence of a "consistent mesoblastic skeleton" in 

 the lophophoral arms, as in Phoronis, and thus forewarned he had comparatively little 

 difficulty in making out " a precisely similar skeleton " in Cephalodiscus. In the latter 

 the so-called " skeleton " of the arms is fixed to the basal apparatus formerly described, 

 and seems to consist of a somewhat firm basement-tissue with longitudinal fibres and 

 reticulations in certain parts. It differs considerably from the condition as figured in 

 Rhabdopleura, in which twisted filaments and particles are described by Lankester. 

 The pinnfe which pass out from the main stem do not taper, and are composed for the 

 most part of granular hypoderm with a few brownish pigment-cells, and the central axis 

 or skeleton. The pigment gives in some a light pinkish or pale violet blush to the 

 feathery plumes, which in life must have been finely tinted ; and it is further interesting 

 that the same pigment occurs in the lophophore of Rhabdopleura, as shown in Professor 

 Lankester's excellent figures.^ The skeleton (PI. IV. figs. 1, 2a) runs from base to apex 

 and terminates within the cellular tip. It is somewhat dilated where it joins the main 

 stem (PL V. fig. 2, sk) and the exact mode of its junction with the axial channel of the 

 latter is difficult to trace, so gradually is it merged into the tissues of the region. 

 No definite ending of these axial structures occurs as in Rhabdopleura, where Professor 

 Lankester figures them as if articulated to the skeleton of the arm, the base of the 

 pinnules dilating, and the central region abruptly terminating, as it reaches the main 

 stem. The dilated bases of the skeletal rods of the filaments in Cephalodiscus join the 

 sides of the reticulated main channel, but no evidence of a continuous central lumen is 

 observed in transverse sections of the free portions, though the double outline, and the 

 appearance of sections of their bases (PL V. fig. 2), would indicate the possibility of such. 

 Endosmosis at least would thus readily occur. In the transverse sections of the bases of the 

 processes just alluded to a series of apertures appears in the tissue of the arm. The 

 condition as described in Rhabdopleura therefore differs from that in Cephalodiscus, 

 especially in regard to the skeleton of the arm, though the general plan of structure is 

 similar. It would also appear to be more readily made out in the former than in the 

 latter, though perhaps this may be partly owing to the examination of fresh examples. 

 Professor Lankester describes the skeleton in Rhabdopleura as cartilaginous, but so far 

 as appearances go in Cephalodiscus it more resembles a structureless translucent basement- 

 substance, probably of a chitinous nature. It was best followed in the preparations 

 immersed in a weak solution of caustic potash. 



The skeleton of the arms and their pinnules gives a definite character to the processes, 

 as observed in the sketches. Though perfectly mobile, the pinnules stand out from the 



' Quart. Journ. Micr. Sci., vol. xxiv., N.S., p. 621. 



