17 



be possible lor the zooid to keep the tip of its stalk, with its series of buds, in the shelter 

 of the coenoecium, and to crawl on the anterior surface of its ])roboscis as far as is necessary 

 tü enable it to add g-elatinous lamellae to the spines and othc;r processes of the coenoecium. 

 A considerable number of Foraminifera and other foreign inclusions occur in various 

 parts of the coenoecium of this species. 



V. EXTERXAL CHARACTERS OF THE ZOOIDS. 



Omitting for the present the males of C. sibogae which must be treated apart (see 

 Sect. XV), all the species of the genus shew the typical Cephalodiscus character, differing from 

 one another only in the relative size and in the shape of the several parts. The zooid consists 

 of the three fundamental regions (I) the buccal shield, with part of its supporting neck, together 

 homologous with the proboscis of Balanoglossus; (II) the collar, produced dorsally into 4 — 6 

 pairs of tentaculiferous arms, and ventrally into the lower lip or operculum ; and (III) the 

 metasome or body, continued into the stalk, which produces buds from its distal extremity. 



Masterman (97, I, p. 290 f.) speaks of the three primary regions as "protomere", 

 "mesomere" and '"metamere", and of their cavities (98, 2, p. 516) as "protocoele", " mesocoeles" 

 and "metacoeles" respectively. Willey (99, 2, p. 314) has pointed out that the use of the term 

 "metamere" in this special sense is inadmissible, and replaces it by "opisthomere". I shall for 

 the most part refer to the three regions as "proboscis", "collar" and "metasome" respectively, 

 the last a term which is used by several authors in describing Phoronis. 



C. dodecalophics (cf. PI. IV\ fig. 42). The general appearance of the zooid is well known 

 from the admirable figure given by M'Intosh (87, PI. II). The proboscis forms an antero- 

 posteriorly ^) flattened disc, which in a front view usually overlaps the whole of the collar 

 except the distal parts of the tentaculiferous arms or plumes, also concealing the mouth and 

 the external apertures of the gill-slits and collar-canals. Its ventral half bears a conspicuous 

 narrow band of reddish pigment, which forms a curved line, with dorsal concavity, extending 

 transversely across the organ "). The body is ovoid and relatively short, the stalk originating 

 from its antero-ventral surface, some way from its aboral end. The short and extremely muscular 

 stalk is usually directed transversely to one side, or towards the mouth. Its epidermis is thrown 

 into numerous, deep, transverse wrinkles, doubtless caused by the contraction of the muscles 

 (see PI. XIII, fig. 169). It seems obvious from this fact that the stalk of the "Challenger" 

 specimens is in a highly contracted condition. It may reasonably be inferred that during life 

 it is capable of a much greater amount of elongation, and it may be presumed that it then 

 frequently places itself in a position parallel to the long axis of the body, with its distal 

 end directed away from the mouth. This position is sometimes actually observed (M'Ixtosh 

 87, PI. III, fig. i); it is the position in which the other species are commonly found; and it 



i) The sense in which I use terms desciiptive of the surfaces and of the principal planes is indicated on p. 23. 

 2) This curved band of pigment has been erroneously represented as tlie mouth in a certain text-book. 

 SIBOGA-EXPEDITIE XWlÓis. 3 



