II. Pelmatozoa. 7 



opening approaches more to the inferior end ; while it loses its connection with 

 the water-tube which is closed all round and lies transversely beneath the vesti- 

 bular fossa between the ectoderm and the hydrocoel. The central mesenchym 

 cells of the stem assume a regular stratified arrangement corresponding to the 

 future stem-joints, and its axis is occupied by a cylindrical cavity enclosing an 

 axial cord, which is solid from end to end and is derived directly from the mesen- 

 chym, not from folds of the peritoneum (Gotte, Perrier). At the time of fixa- 

 tion and metamorphosis the ciliated bands disappear. The great ventral fossa is 

 invaginated to form the tentacular vestibule which is soon closed again with a 

 thin roof and thick floor. The latter and a great part of the former are derived 

 from the superior lip of the original depression. It caps the visceral mass at the 

 superior end of the elongated larva, while the mass itself abandons its transverse 

 position for a more longitudinal one. The intestine swells into a large thin 

 walled sac, filled with a granular matter which is destined for ultimate resorption 

 (vitellus nutritif); while the water-tube also increases in size and acquires 

 communication with the exterior by a slender tube which opens on the left side. 

 The pore sometimes appears sooner, before metamorphosis commences. At the same 

 time the left peritoneal sac inserts itself between the right sac and the hydrocoel, 

 and so gives rise to the horseshoe-shaped oral coelom, the space between the 

 free ends of which is occupied by the water-tube. Fixation is accompanied by 

 a gradual change in the ectoderm, the cells of which become separated from one 

 another by a collagenous material and unite by internal processes with the stellate 

 cells of the general cavity (schizocoel), to which they acquire a considerable 

 resemblance. Hence the whole body-wall seems to consist of interlacing fibrils, 

 the outermost of which have a certain regularity of arrangement. The 5 primary 

 tentacles become lobate and push before them the funnel-shaped floor of the vesti- 

 bule which is at first undivided ; and the vestibular cavity becomes differentiated 

 into a gullet and an intratentacular space. The right end of the horseshoe-shaped 

 oral coelom bends down to form the columellar cavity, which occupies the mid- 

 ventral line with the water-pore to its left, while the anus appears on the right. 

 The hydrocoel becomes reduced in size with thinner walls, and the water-tube 

 increases in width, exchanging a cylindrical for a flattened epithelium, and is tra- 

 versed, like the oral coelomic space, by trabeculae. It gradually becomes covered 

 by the ends of the horse-shoe formed by the hydrocoel and the oral coelom, while 

 the slender canal joining it to the water-pore increases in length, passing between 

 the oral coelom and the cortical zone. Where it traverses the latter to reach the 

 water-pore it expands into an ampulla. The columellar cavity now increases in 

 size and penetrates as a deep sinus into the endodermic mass, which thus becomes 

 divided into a stomach on the left and the rectum on the right ; while the axial 

 cord of the stem extends upwards in this sinus between the gullet and the oral 

 coelomic ring to form an annular cord above the latter. The inner wall of the 

 columellar cavity is reflected over it, and the organ thus formed becomes the ovoid 

 gland. The cellular coat of the endodermic mass now undergoes a gradual 

 thickening which extends outwards from the bottom of the notch enclosing the 

 columellar cavity, and when 25 tentacles have appeared, the rectum opens to the 

 exterior at a point on the mesentery between the oral and aboral divisions of the 

 coelom, and at the angle where the former bends down to form the columellar 

 cavity. About the same time the oesophageal invagination joins the stomach. 

 The 5 primary sac culi are formed at the expense of the outer wall of the water- 

 vascular ring, and are comparable to the 5 rudiments of the otocysts in theSynapta- 

 larva. They are certainly not parasitic Algae, as supposed by Vogt & Yung [see 

 Bericht for 1S86 Ech. p 6 ; and 1887 Ech. p 5]. In some abnormal larvae 



