646 PROFESSOE E. RAY LANKESTER. 



ing the floor of the untenanted chamber without giving off any bud. e. Purple- 

 coloured material, such as is often seen occupying an empty chamber, and 

 probably due to the breaking down of the tissues of a bud or polypide en- 

 closed in the unopened chamber. 



Fig. 3. — Portion of the stalk from such a branch as that drawn in PI. 

 XXXIX, fig. 3, taken about the region m, showing the gradual conversion of 

 the soft-walled brilliantly-coloured gymnocaulus into the hard dark-brown 

 pectocaulus. A thin cuticle, the commencement of the caulotheca or stalk- 

 pipe, already invests the portion of stalk here drawn, but the coloured epidermal 

 cells show through it. 



Fig. 4. — A further stage in the conversion of the gymnocaulus into pecto- 

 caulus. 



Fig. 5. — Completion of the change. The caulotheca is now so dense that 

 the pigment spots of the enclosed stalk are no longer visible. The stalk has 

 shrunk much in bulk, and the stalk-pipe (caulotheca) has shrunk and hardened 

 at the same time, and is now embedded in laminse of the tubarium (see 

 Figs. 9 and 10). 



Fig. 6. — Portion of the axial tubarium, showing a small closed chamber 

 containing a spherical bud {g) apparently in healthy condition (? a hybernacu- 

 lum). a,b,c. Successive chambers of the tubarium. d. Pectocaulus. e. 

 Wall of the tubarium in optical section. /. Stalk of a polypide not further 

 shown in the drawing, g. The spherical bud, or hybernaculum. h. Grains 

 of siles enclosed in the tubarium. N.B. — In this and Fig. 7 the laminar struc- 

 ture of the septa of the tubarium are well seen. 



Fig. 7. — Three polypides, c, e, g (of which the last is still a very young bud), 

 given off nearly simultaneously from the axis .ry. The polypide stalk (gymno- 

 caulus) of the polypides e and / are supported each on an unusually long 

 pedicle of pectocaulus, h^ and h^. Septa divide the chambers a from b, and b 

 from c, d, and e. 



Fig. 8. — Gastric epithelium of Rh. Normani. 



Fig. 9. — Transverse section across the axis of a colony of Rhabdopleura 

 Norma ni. Owing to the strong contraction of its stalk a polypide has been 

 withdrawn into the axial chamber, and is seen in section, a. Test of the 

 Ascidian, to which the tubarium is adherent. B. Substance of the tubarium 

 (coloured yellow), c. Cavity of the axial chamber, d. Caulotheca, stalk- 

 pipe, or cuticular investment of the pectocaulus embedded in the substance of 

 the tubarium. e. Cavity of the pectocaulus, due either to shrinking of the 

 soft tissue away from the cuticular investment, or possibly a normal space 

 between the pectocaulus and its sheath. /. Soft substance of the pectocaulus, 

 showing pigmented epidermic cells, g. Epidermic pigmented cell. h. Intes- 

 tine, i. Stomach, k. Muscular tissue of body wall. I. Thoracic origin of 

 the axial skeletal cord of the polypide stalk, m. Body cavity (ccelom). 



Fig. 10. — Transverse section further along the same axis. The whole area 



