MUSEUM OF COMPARATIVE ZOOLOGY. 59 



wall of the ring canal is two-layered, the tentacles are two-layered also. 

 The outer layer of the tentacle is thus derived from the inner layer of 

 the bud ; the inner layer, on the contrary, from the outer layer of the 

 bud. It would be hardly necessary to make this statement, which 

 agrees both with early and the most recent observations, had not Bar- 

 rois ('86, p. 75, Fig. 48) referred to and figured the tentacles as having 

 been formed from the inner layer of the bud only. 



My observations fully confirm Seeliger's ('90, p. 587) description of 

 the manner of growth of the tentacles ; that is, that the outer edge of 

 the ring canal, together with its tentacles, moves downward and outward 

 along the sides of the polypide, turning the axis of the tentacle from a 

 nearly horizontal to a vertical position, and increasing the area of the 

 kamptoderm. Thus in Figure 92 this process has progressed farther on 

 the left side than it has on the right. 



Nitsche ('71, p. 458) lays some stress upon the statement that the ten- 

 tacles are not at first few in number, gradually becoming more numer- 

 ous ; on the contrary, he says, " Ich sah stets, beim ersten Auftreten von 

 Tentakelanlagen, 16, 17, oder 18 Stiick gleichtzeitig erscheinen." See- 

 liger ('90, p. 584) agrees with Nitsche in this respect ; but Prouho ('90, 

 p. 449) finds the conditions diflTerent in Flustrella. Here the tentacles 

 " ne se developpent pas simultanement sur tout son pourtour, mais ap- 

 paraissent d'abord de chaque cote du plan de symetrie, puis se multi- 

 plient vers I'ai'riere." As I have shown, 14 of the 17 tentacles arise 

 nearly simultaneously in Paludicella, for here there are few of them ; 

 and this is the case also in Escharella variabilis with its 17 tentacles. 



As the tentacles of both Flustra and Bugula are few in nuraber,^ the 

 statements may easily be considered to be correct for these genera. The 

 tentacles of Flustrella hispida are much more numerous (30-35), and 

 Prouho's statement may well be true for his form. In fact, my own ob- 

 servations on this species are fully in accord with those of Prouho. 

 Figure 77 (Plate IX.) represents a young polypide of a Flustrella corm, 

 viewed from the roof as an opaque object. Six tentacles were visible on 

 each side of the bud, but the oral and anal parts of the corona were yet 

 incomplete. The remaining nine or ten pairs of tentacles subsequently 

 arise oralward and analward of these rudiments. 



Much disagreement has prevailed concerning the number of layers in- 

 volved in the kamptoderm of marine Gymnoleemata, in both the adult 

 and the developmental stages. As in so many other cases, we owe to 



1 Bugula avicularia has 14 or 15 tentacles, and Flustra (Membranipora) mem- 

 branacea 20, according to Hincks ('80, pp. 76 and 140). 



