MUSEUM OF COMPARATIVE ZOOLOGY. 147 



this is a zone (z. pap. a.) with small papillae; this measured 3 mm. in 

 breadth in the same specimen. The papillae of this zone appear super- 

 ficially as minute discoidal elevations of the skin. In well expanded 

 specimens, the tentacles droop over and nearly cover this zone, which is 

 not separated from their base by any definite line, since the papillae 

 extend forward a short distance over the aboral surface of the tentacles, 

 becoming gradually less frequent. They are indeed met with occasion- 

 ally on the whole of this surface, but are entirely wanting on the oral 

 aspect of the tentacles. In all well expanded specimens these regions 

 are as well defined as in the one which has served as the basis for this 

 description, and the zones have the same relative size as in the measure- 

 ments given. 



2. Tentacular Fold. 



The tentacles (Tentakelmembran) originate in the larva as two folds 

 of the oral margin, — " lippeuartige Falten," Hatschek ('83, p. 115), — 

 separated dorsally but continuous ventrally, and lying right and left of 

 the median line. Starting, then, from this primitive condition, the form 

 found in the adult would be reached, if it be supposed that these flaps 

 of skin are plaited radially to the oral centre, and that the growth is 

 more rapid on the oral surface as well as toward the margin, thus ne- 

 cessitating a reflection of the flaps back upon the aboral surface. For 

 a careful examination shows that in well expanded specimens the so- 

 called tentacles consist of a thick fold of skin surrounding the terminal 

 oral orifice with numerous plaits and folds arranged radially. This 

 continuous flap may be called the tentacular fold, in preference to 

 membrane, since the latter suggests a false idea of its nature, and its 

 subdivisions may conveniently be termed the radial plaits. 



The general form of the tentacular fold, as viewed from above (Fig. 2), 

 may be said to be that of a horseshoe with the smaller dorsal curvature 

 interrupted on the middle line. The external or ventral semi-circum- 

 ference is reflected over the superior portion of the introvert, whereas the 

 internal or dorsal portion makes a ventral flexion over the mouth, and lies 

 higher than the other half of the tentacular fold. The superior height 

 of the dorsal portion of the flaps in the larva caused Hatschek ('83, p. 115) 

 to regard this as the " Anlage " of the first pair of tentacles. He knew 

 nothing, however, of the further development of this portion, which 

 probably represents the origin of the dorsal horns, since separate tenta- 

 cles do not exist. In the adult, at any rate, this region shows two horns 

 (Figs. 1 and 2, crmi. d.) projecting ventrad over the oral aperture, and 



