PLATE XII. 



Figs. 141 — 157. — C. dodecalophus. Figs. 158 — 160. — C. levinscni. 



Figs. 141 — 151. C. dodecalophus. — Obliquely sagittal sections, through the right half of the anterior end. 

 Every second section is drawn, beginning with fig. 141, which is the most external. Fig. 151 

 is more hic^hly magnified than the ethers. The series illustrates the rclation of the arms and of the 

 operculum to the rest of the collar, together with the more important parts of the nervous system. 

 The muscles, and the filaments traversing the collar-cavity are only partially represented. 

 — Shews the lateral part of the dorsal lobe of the proboscis (/>.) and the six arms (/ — 6) of the 

 right side. The longitudinal muscles are shewn in 5 and 6 only. 



The lateral lobe of the operculum {op.r.) is cut tangentially. Arm / is still distinct, but 2 — 5 



have passed into the common arm-base. 



The ventral lobe of the proboscis is beginning to appear, with the end of the pigment-band. The 



arm-base is traversed by food-grooves, continuous with those of the arms. Grooves j — (5 are passing 

 transversely inwards. The collar-cavity in the arm-base is interrupted by projections of the basement- 

 membrane which are the first indications of the separation of the arms from the common base. 



Pier. \AA — The first arm has joined the arm-base. The dorsal sides of the arms are marked A'. / — 6; and the 

 crrooves / — 6. Grooves 3 — 5 are shallower than in the last figure. The coelom of the part of the arm- 

 base corrcsponding with arms j — 6 has become continuous. A fold of the dorsal wall of the proboscis 

 is cut tangentially, and the nerve-layer is accordingly thicker here than in ether parts of the section. 



Fio-. 145. — Grooves 4., 5 have quite disappeared, the anterior surface of the arm-base [a.b.) being here smooth. 



Fig. 146. — The dorsal portion of the collar is beginning to be connected with the proboscis. The tip 

 of the right anterior horn of the collar-cavity is seen projecting into the proboscis-cavity. 



Fio-. 147. — The first arm [R. i) is cut at its base, its cavity communicating with the anterior horn of the 

 collar-cavity. The two parts of the collar are about to become continuous: — P-P->'-< external 

 aperture of right proboscis-pore. 



Fig. 148. — A lobe of the operculum (op.r.) which is continuous with the base of the sixth arm, projects 

 backwards, in the plane of the section, over the collar-pore, which is seen in fig. 151 [cc. e.). 

 The septum on the dorsal side of op. r. is a ridge of basement-membrane, indicating the point 

 at which the operculum becomes free from the rest of the collar (cf. figs. 152, 153): • — p.p.r., 

 right proboscis-pore, cut tangentially; per., pericardium. 



Fig. 149. — Cuts the animal at the level where the collar first joins the body: — ovd., right oviduct, 

 with the ovarian vessel [ov.v.) and mesentery [ov.in.]; ines.^, dor.sal mesentery, on the other side 

 of which is seen the left ovary [ov.l.) and the left lateral edge of the rectum (;-.). 



Fig. 150 shews the origin of the lateral nerve (/.«.) from the central nervous system [c.n.s.]: — div., edge 

 of the pharyngeal diverticulum ; ovd., right oviduct;/././., internal opening of left proboscis-pore; 

 b.c?a., tip of anterior horn of left collar-cavity. 



Fig. 151 cuts the tip of the notochord («c/^), which is supported by the dorsal mesentery of the collar;/./. /., 

 external opening of left proboscis-pore; c.n.s., ganglion-cells of central nervous system; ov.l. and 

 ov.r., left and right ovaries, with their respective mesenteries [ov.m.l., öï^.w/.r.) ;/.«., lateral nerve; 

 C.C. e., external aperture of right collar-pore, with its problematical tissue (.r.); or.m., oral muscle. 



Figs. 152 — 157. — Frontal sections of a young individual, passing from the dorsal side towards the ventral 

 side. The figures illustrate the arrangement of the oral grooves, the dorsal diverticulum of 

 the pharynx, the pleurochords, the gill-slits and other organs. 



Fig. 152, near the dorsal extremity of the pharyngeal diverticulum [div.), with its glandular patches, cuts the 

 right lateral lobe (op.r.) of the operculum tangentially: — .§-/., ventral end of glomerulus, attached 

 to the anterior side of the notochord (i/cli.); x., dorsal end of problematical tissue of right collar- 

 pore, attached to the epidermic fold which separates the lateral lobe of the operculum from the 

 rest of the collar; g.p., right generative pore; R..^ — ó, groove on the ventral surface of the 

 common arm-base leading to the food-grooves of the .^"' — ö"' right arms. The epithelial ridge 

 (i'.r.) seen in this figure and in figs. 153 — 155 is discussed on p. 62. 



F'S- ^53» through the left lateral lobe [op.l.) of the opercidum, shews the dorsal ends of the pleurochords 

 (//.) ; or. ?«., oral muscle. 



Fig. 154. — The pleurochords (//.) open into the diverticulum of the pharynx, on either side of its posterior 

 groove. The right collar-canal (c.c.r.) lies in a direction which is seen to be at right angles to 

 that of the corrcsponding gill-slit (g.s., fig. 156). 



ï^ig- 'SSi through the dorsal edge of the mouth, shews a labial or oral groove (l.g.) between the base of the 

 operculum (op.l.) and a ridge (e.r.) which is prolonged dorsally as the epithelial ridge visible in 

 figs. 154 — 152. The oral groove communicates with one of the anterior grooves of the pharyngeal 

 diverticulum. The right pleurochordal region is about to open by the gill-slit, the dorsal wall 

 (g.s.) of which is cut tangentially; cc, ventral wall of right collar-canal. 



Fig. 156 shews both gill-slits (g.s.), the ventral wall of the left collar-canal (e. r.), and the anterior horns of the 

 third body-cavity (iJ. r.^rt.), filled with longitudinal muscles. These structures just extend into fig. 155. 



Fig. 157 shews that the pleurochords (//.) extend back laterally along the pharynx. The operculum (op.) is 

 about to terminate in the middle line, as is indicated by the absence of body-cavity in this region. 



Figs. 1 58 — 160. — Diagrams of the proboscis and collar of C. levinseni, based on the specimen illustrated in PI. X. 



Fig. 158. — Diagrammatic posterior view of proboscis (/.), collar (c) and dorsal part of metasome (met.); 

 P-P-, proboscis-pores; L. i — 6, arms of the left side; op. operculum. 



I'ig- 159- — Diagrammatic anterior view of a specimen in which the proboscis-stalk is directed dorsally, and 

 the buccal disc has been removed by a cut transverse to the proboscis-stalk. The operculum (op) 

 is supposed to have been stretched so as to pass completely outside the series of arms (cf. fig. 160): 

 L.i—6, food-grooves of the left arms; the body-cavity of the left arm-base is partially divided 

 by ingrowths of the basement-membrane, corrcsponding with the intervals between the arms. 



Fig. 160. — A similar diagrammatic view, with the proboscis in" its usual position. 



[Figs. 141— 150, 132—1 s-j were draivn ivith Zeiss, C Obj., fig. iji with Zeiss, D D. All the figures zvere 

 reduced '/.i]- 



