P O L Y Z A 



165 



surface by which the tube or case (tubarium) in which the 

 polypide is enclosed is secreted. 



Both Rhabdopleura and Cephalodiscus produce colonies 

 by budding; but the colonies of the former are large, 

 definite, and arborescent, whilst those of Cephalodiscus 

 are remarkable for the fact that the buds do not remain 

 long in organic continuity with their parent, but become 

 detached and nevertheless continue to be enclosed by the 

 same common envelope or secretion. The bud-formation 

 of Rhabdopleura recalls that of Paludicella in the fact that 

 it leads to the formation of continuous arboriform com- 

 munities. That of Cephalodiscus resembles the budding 

 of Loxosoma, since no two fully-formed individuals remain 



'.I 



FIG. T. Rhabdopltura yormani, Allman (original drawings, Lankester). A. 

 A single polypide removed from its tube and greatly magnified, a, mouth: b, 

 anns ; c, polypide-stalk or Kymnocanlus, the ' contractile cord " of Sara ; d, the 

 prae-oral lobe (buccal shield or disk of Allman); t, intestine: /, thoracic 

 region of the polypide ; g, one of the ciliated tentacles. B. Lateral view to 

 show the form of "the buccal shield and its pigment spot, g, ciliated tentacle 

 (in outline) ; h, basal ridge of the right arm of the lophophore. C. Lateral view 

 of a polypide. i, ciliated patch (Sars's organ) at the base of the lophophore-arm. 

 Other letters as above. D. Part of a lophophore-arm. with soft tissues rubbed 

 off to show the cartilaginoid skeleton, a, epithelium and soft tissues still 

 adherent at the tip of a tentacle; 6, skeleton of tentacle; e, skeleton of axis. 

 E. Portion of a colony of Rhabdopleura A'ormani, showing the branched tube- 

 like cases formed by the polypides. The black line within the tubes represents 

 the retracted polypides connected together by their common stalk, the pecto- 

 caulus. Magnified to three times the size of nature. 



in organic continuity. Both Rhabdopleura and Cephalo- 

 discus (like Phoronis) produce cases or investments in 

 which they dwell. These are free secretions of the organ- 

 ism, and are not, like the ccenoecia of Eupolyzoa, cuticular 

 structures adherent to and part of the polypide's integu- 

 ment. The dwelling of Rhabdopleura is a branched 

 system of annulated tubes of a delicate membranous con- 

 sistency, each tube corresponding to a single polypide, the 

 rings of which it is built being successively produced at 

 the termination of the tube by the secreting activity of the 

 pree-oral disk (fig. 7, E). The polypides freely ascend and 

 descend in these tubes owing to the contractility of their 

 stalks. On the other hand the dwelling of Cephalodiscus j 



is a gelatinous, irregularly branched, and fimbriated mass 



(fig. 8), excavated by numerous cavities which communicate 



with the exterior. In these 



cavities are found the nu- 



merous detached small 



colonies of Cephalodiscus 



(fig. 9), or we should rather 



say the isolated budding 



polypides. The remaining 



important feature in the 



organization of the Ptero- 



branchia, namely, the parts 



connected with the forma- 



tion of buds, are best un- 



derstood by first examining 



Cephalodiscus. The body 



of Cephalodiscus is seen 



(fig. 9) to be an oval sac ; 



in this is suspended the 



U-shaped alimentary canal, 



and from the -walk of its 



cavity (ccelom) the ova and 



the spermatozoa are de- 



veloped. Projecting from 



the ventral face of this 



oval sac is a muscular cy- 



lindrical stalk, into which 



the viscera do not pass, 



though the ccelom is con- 



tinued into it (fig. 9, c). 



This stalk is merely the 



outdrawn termination of 



thp hoHv Tr i ariont as FIG. 8. Dwelling of gelatinous consistence 



DO -!- ' and brown colour formed by the polypides 



long as the whole of the of Ccphalodiiau dodtmlojilwu, M'Intosh; 



t ,, i i -. natural sire (from an original drawing 



rest of the animal, and it kindly supplied by Prof. M-intosh,F.E 



. 



is from its extremity that o. polypide within the jelly;^, cavity once 

 , , , *, ._ occupied by polypides. 



the buds are produced (fig. 



9, a). Before the buds have attained half the size of their 

 parent they become detached, but continue to occupy some 

 portion of the common gelatinous dwelling. 



FIG. 9. A polypide of Cephaloditmt dodecaiophut removed from the gelatinous 

 bouse (from an original drawing by Prof. M'lntosh). No organic connexion 

 has been severed in thus isolating this polypide with its attached bods a, a, 

 The figure represents the furthest point to which colony-formation attains in 

 this form, a, buds growing from the bas of the polypide-stalk ; 6, the pne- 

 oral lobe (bnccal shield or disk); c, the polypidc-stalk ; </, the ciliated tentacles 

 of the twelve lophophore arms (six pairs, each like the single pair of Rhab- 

 dopleura) inextricably matted and confused ; e, anterior margin of the pre- 

 oral lobe ; /, posterior margin of the same. Magnified about fifty times linear. 



Turning to Rhabdopleura, we find that each polypide 

 has a body of similar shape and character to that described 

 for Cephalodiscus, and a similar ventrally developed 

 " stalk :l (fig. 7, A, c). But, inasmuch as the buds deve- 



