282 PROCEEDINGS OF THE AMERICAN ACADEMY 



more frequently at the edges of the fans than elsewhere. 6. The mar- 

 ginal branches are shortest, the middle ones longest. 7. There is one 

 proximal individual to each "fan." This is followed by two, and these 

 by four, of which four the two inner adjacent are lateral, the two outside 

 ones terminal. Then each of the two outside individuals of these four 

 bears more individuals, countuig all which are formed between it and the 

 periphery, than does each of the inner individuals. 8. New individuals 

 are constantly being formed at the periphery of the fan, and at about 

 the same time, but on some branches only one new bud arises, on others 

 two. The tendency to give rise to two buds decreases as the fan 

 grows older ; and if a number of arcs be struck across an accurate 

 drawing of an entire fan, with the proximal individual as a centre 

 and with different radii, it will be found that the number of individ- 

 uals cut by any linear unit of arc is the same for all radii. 



In Bugula flabellata the fans are not attached to an erect axis, 

 but are each attached to the rock or woodwork by their proximal indi- 

 vidual. Three or five branches are united together, instead of two, as 

 in B. turrita. The above " laws " are equally applicable to this spe- 

 cies, except that No. 4 does not apply well here, being masked by 

 another, namely, that of the three or five branches wliich are united 

 together the outer ones only give rise to lateral buds. The above 

 rules hold for Crisia eburnea also, which rises erect like Bugula, and 

 has its branches united in pairs. 



In genera which, like Membranipora, Lepralia, and Escharella, form 

 creeping colonies in which all of the branches cling together, the 

 normal architecture of the colony is obscured by inequalities of the 

 surface upon which it lies. But under favorable conditions there is a 

 tendency to conform to the laws which we discover in Bugula. 



Regeneration of polypides has been studied in Escharella and Flustra. 

 In these cases regeneration occurs at one point only ; namely, on the 

 operculum immediately behind, i. e. proximad of the atrial opening. 

 Regenerated buds thus arise in the immediate vicinity of older ones, 

 and from those cells some of which went to form that older polypide. 

 They are formed by an invagination of the body-wall exactly as are 

 the old polypides. Although the cells of the operculum have lost 

 their cuboid form, and only return to it again in giving rise to the 

 new bud, yet the nuclei appear to remain more abundant here than 

 elsewhere on the body-wall, and this may perhaps be considered a 

 condition of less extensive differentiation than obtains in all other 

 parts of the body-wall. 



Cambridge, February 9, 1891. 



