REPORT ON CEPHALODISCUS DODECALOPHUS. 5 



spinous processes arc very large, and project far beyond the others, while occasionally 

 they occur in groups. They generally taper a little towards the tip, which is often 

 attenuate, and of a deeper brownish hue than the rest of the coencecium (PI. VII. fig. 1). 

 The free tips of the branches frequently show a somewhat palmate arrangement, with 

 longer spines variously divided. The irregularity in regard to the distril^ution of the 

 spines recalls the processes on the peculiar sponge Chondrocladia, though this feature 

 is much more marked than in the latter. All the spines are hollow, and in connection 

 with the canals and cavities of the coencecium. 



The surface of the coencecium, moreover, is dotted, especially at the bases of the 

 spines, with large rounded apertures, which lead into the interior of the stem, the latter 

 being honeycombed from end to end by an irregular system of wide canals and somewhat 

 rounded cavities, intersected by bridles and arches, which thus provide for the constant 

 ingress and egress of sea-water throughout the entire system. The inner wall of these 

 canals and chamlDcrs is as smooth and glistening as the outer surface of the coencecium, 

 the secretion being perfectly homogeneous. It cuts with great readiness, and as cleanly 

 as a soft Fucus ; while it is much less tough than the glistening tubes of the Annelids. 

 Microscopically it is composed of numerous layers of a translucent and very fine mem- 

 branous secretion, so that in the preparations there are endless lines and folds, while the 

 sheen or lustre is doubtless due to the same arrangement.^ The whole disposition of the 

 tissue clearly indicates that it is the work of the polypides, just as much as the tube of 

 an Annelid or Phoronis, the more regular and less bulky tube of Rhahdopleura, and in 

 some respects the shell of a MoUusk. Like the Annelidan tubes it most approaches, it 

 is little affected at first either by nitric acid or caustic potash, though the former after a 

 time somewhat softens and bleaches it. 



This secretion of Cephalodiscus is paralleled by the curious investment or " house " 

 of Appendicularia, which by some has been held to be the homologue of the Ascidian 

 test, and which fills the tow-net with a semi-solid mass when the animals are abundant. 

 It difi"ers considerably from the branched system of annulated tubes formed Ijy Rhah- 

 dopleura, each of these corresponding to a single polypide, while the rings of which it 

 is composed are successively produced at the termination of the tube by the secreting 

 powers of the great buccal shield or prae-oral disk. Professor Lankester, moreover, has 

 very clearly explained^ that the differences of the rings in the attached or recumbent part 

 of the tube and those of the erect portion — dififerences first pointed out and figured by 

 Professor Allman ^ — are due to the changes in the buccal disk which secretes them, this 

 disk being characteristically bifid in the young specimens which form the recumbent 

 portion of the tube. This symmetry and regularity are absent in the house of Ccphcdo- 



1 All these features are well seen in a series of sections mounted by the dextrous hands of the late Professor Busk. 



2 Quart. Journ. Micr. Sci., vol. xxiv., N. S., p. 625. 



^Journ. Roy. Micr. Soc, vol. ix. pp. 61-02, pi. viii. figs. 4, 6, &c., 1869. 



