II. Pelmatozoa. 9 



remainder of the anterior body-cavity extends forwards as a narrow cavity lined 

 by flattened cells. By the 7 th day the right pouch extends forward into an axial 

 cord of mesoderm cells within the pre-oral lobe, as 5 minute cavities arranged 

 round a central strand or axial organ in a horse-shoe which is open on the ven- 

 tral side. These 5 cavities are the rudiments of the chambered organ. - 

 The basal and oral plates appear early, on the 6 tL day, in two horse-shoes open 

 ventrally and generally parallel to the mesentery between the right and left body- 

 cavities, the basals lying round the right and the orals round the left cavity. The 

 dorso-central and 3 stem-joints appear about the same time, the latter being 

 imperfect rings which are open ventrally. They close after fixation and surround 

 the cavities of the chambered organ. New joints continue to appear, and early 

 on the 7 th day 3 (rarely 4 or 5) underbasals are formed, 2 of them being larger 

 than the third, which lies in radius A [Bury here follows Carpenter's nomenclature 

 of the radii of an endocyclic Crinoid. When the disc is upwards and the anal inter- 

 radius towards the observer, the ray opposite to it is called A, and the others B, C, 

 D, E in succession following the coil of the gut (and the watchhand)], opposite to 

 the interradius of the water-pore (C D). The larvae are hatched on the 7 th day 

 after fertilisation , swimming for 1 2 hours and then attaching themselves by 

 the pre-oral pit, which rapidly flattens out into a disc. The anterior tuft and 

 first band of cilia disappear very soon, the other bands remaining active for a 

 few hours. Histolysis spreads through the tissues setting inwards from the ecto- 

 derm, while the larval mouth flattens out and disappears , 2 lateral folds gro- 

 wing across it towards one another. Closure proceeds from behind forwards 

 and gives rise to the vestibular cavity which extends rapidly backwards till 

 its floor envelopes the hydrocoel ring and fuses with a mass of minute nucle- 

 ated cells, which have been proliferated into the gut from the centre of the ring. 

 All the internal organs are now rotated towards the posterior or oral pole, 

 so that the vestibule and hydrocoel ring assume a transverse position. The 

 anterior body-cavity (water-tube, Barrois; parietal canal, Perrier) now lies 

 entirely in the body wall and is no longer continued into the pre-oral lobe, but 

 is situated near the oral pole in the interradius of the water-pore (C D) . In this 

 Cystid phase the oral or left body-cavity forms a longitudinal mesentery, in 

 the same interradius which supports the water-tube ; while the aboral or right ca- 

 vity forms an oblique longitudinal mesentery, joining the stem in interradius B C. 

 The 5 hydrocoel lobes become trifid, forming the rudiments of 15 tentacles, while 

 its anterior blind end grows outwards in the oral longitudinal mesentery as the 

 water-tube or stone canal, which is lined by the same high epithelium as the 

 hydrocoel itself. On entering the body wall it turns slightly towards the oral sur- 

 face and finally joins the parietal canal, thus placing the hydrocoel in communi- 

 cation with the exterior through the water-pore. The parietal canal has a flatter 

 epithelium than either water-pore or water-tube. The mesenteron now becomes 

 crescentic, its blunter horn remaining connected with the floor of the vestibule 

 and ultimately being hollowed to form the stomodaeum which lies in interradius 

 C D. The other horn which is more tapering runs in the transverse mesentery to 

 join the body wall in radius C, and ultimately opens to the exterior as the rec- 

 tum, usually in interradius CD. Soon after the Cystid stage is reached the 

 chambered organ becomes completely shut off from the coelom by transverse par- 

 titions ; while the axial organ in its centre grows upwards in the aboral longi- 

 tudinal mesentery which it leaves near the oral surface to push its way beneath 

 the peritoneal epithelium of the splanchnopleure. This forms a sheath over it 

 which runs up to the floor of the vestibule, though the axial organ itself ends by 

 the side of the gullet at the level of the transverse mesentery. The sacculi 



