ON THE DEVELOPMENT OF TUBULIPORA. 113 



careous part. I regard these as post-mortem changes due 

 to the action of reagents, and I think it probable that in the 

 healthy zooecium or ovicell the distal end of the body-wall is 

 in the immediate neighbourhood of the edge of the calcareous 

 wall which it secretes. 



The pupil-like opening seen in the terminal membrane of 

 living colonies is doubtless the external opening of the vesti- 

 bule. When the membrane is in its proper place it is probable 

 that the body-wall of this region is of no great thickness ; but 

 in the more usual retracted condition, which is probably arti- 

 ficial, the irisoid (adopting Jullieu^s name), or in other words 

 the terminal membrane of the zooecium, generally appears as a 

 thick mass of nucleated protoplasm. 



The young zooecium with its polypide-bud has been seen to 

 consist of the following parts (cf. 15, pi. xxii, fig. 1) : — an ex- 

 ternal terminal membrane (irisoid), a vestibule, and the two- 

 layered vesicular bud. Further examination has shown, how- 

 ever, that the outer layer of the bud is reflected at its distal 

 end into a thin membrane (fig. 33, s. m,), which encloses the 

 whole bud, a cavity occurring between it and the bud. In 

 older polypides the alimentary canal, the retractor muscles, 

 and the reproductive organ, whether ovary or testis, lie in this 

 space, which is the body-cavity. The reflected layer seen in the 

 young bud may probably be regarded as the somatic mesoderm. 

 This perhaps throws some light on the morphology of the 

 cavity lined by the inner layer of cells of the secondary embryo 

 (cf. 15, p. 223, pi. xxiv^ fig. 23j. The inner layer applies itself 

 closely to the dorsal ectoderm in the region which will give 

 rise to the primary polypide bud. If that bud were developed 

 by an invagination of the entire dorsal wall, and the shape of 

 the cavity of the inner layer were simplified by the eversion of 

 the sucker, the cavity in question would closely resemble what 

 has just been described as the body-cavity of the bud. 



b. Excretory Vesicles. 

 The most casual inspection of sections of the three species 

 of Tubulipora which I have specially studied reveals the 



VOL. 41, PART 1.— NEW SEE. H 



