486 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



them. Like the fibrillse these cells, which eventually will become ganglion cells, do 

 not appear simultaneously throughout the whole extent of the ring. 



At this stage cordlike processes from the nerve ring extend toward the bases 

 of the tentacles, where in the single-layered ectoderm a few fibrillre can be 

 detected in the inner ends of the cells. These can not, as a rule, be followed 

 farther into the tentacles, though preparations consisting of the whole animal 

 sometimes show a fibrous cord running closely beneath the ectoderm which is prob- 

 ably nervous. 



In the free-swimming larva, besides the nerve layer beneath the apical pit, 

 there are two lateral nerve stems which run along either side of the vestibular 

 groove. It could very well be that, as a result of these changes in the vestibule, 

 these might come to occupy the same position as the later nerve ring. But appar- 

 ently no such transformation occurs, the nervous system of the free-swimming 

 larva disappearing in its entiety. 



ECTODERM OF THE TENTACLES. 



At the close of the developmental period last described there were 25 tentacles, 

 each covered with a similar ectodermal epithelium. 



After the rupture of the vestibule the tentacles enlarge very rapidly and 

 become movable to the highest degree. 



In the oldest larvae, up to the time of the appearance of the rudiments of 

 the arms, the first 15 tentacles are immediately distinguishable in size and struc- 

 ture from the 10 interradial. The former were called by Sir Wyville Thomson 

 the " extensile tentacles," the central one being the " azygous tentacle," and the 

 latter the " nonextensile tentacles." 



The number of interradial tentacles does not remain limited to 10 but increases 

 through the appearance of new tentacles interradially between those already 

 formed indiscriminately in the several radii. Occasionally new tentacles appear 

 between the larger radial tentacles also. The highest number of tentacles observed 

 by Seeliger at this stage was 29, 4 accessory tentacles having arisen. As pre- 

 viously, the 15 older tentacles stand farther from the center of the animal than 

 the 10 smaller and the accessory. 



While the 15 older tentacles are entirely independent of each other throughout 

 their entire length, being only in contact at the base, the two interradial tentacles 

 in each interradius are united across the interradial planes by an ectodermal fold, 

 whereby the independent mobility of each is inhibited. 



These folds reach from the bases of the tentacles to somewhat beyond their 

 middle. On their distal border they appear considerably thickened, so that on 

 preparations made of the entire animal at first only this thickened edge is seen, 

 stretching rafter-like between the two tentacles. But on sections it is easy to see 

 that this is an ectodermal fold of which the inner wall passes over laterally into the 

 tentacles and basally into the periphery of the oral disk at the place beneath which 

 the nerve ring lies, while the outer, more strongly flattened, curls around into the 

 inner wall of the oral flaps and is laterally produced into the ectodermal epithelium 

 of the tentacles. 



