Compensatory Regulation. 39 



operculum upon the rudimentary one which prevents the latter 

 from attaining its full development. The removal of the func- 

 tional operculum removes the restraining influence and the rudi- 

 mentary continues its development. The nev^^ functional oper- 

 culum in turn restricts the new bud developing in place of the 

 old functional operculum and holds it at the rudimentary stage. 

 The plausibility of this explanation is increased if it is found 

 that in the ontogenetic development one operculum develops 

 before the other, and therefore can hold the latter in check in 

 the manner before indicated. With this object in view the 

 investigation of the ontogenetic development w^as undertaken. 



h. Historical Revieiu. The first recorded observations on the development of 

 the branchial apparatus in Serpulids which I have been able to find are those of 

 Milne-Edwards ('45), on the development of the young Protula. He saw the larvae 

 attach themselves to solid objects at the bottom and sides of his dish. Here they 

 secreted a cylindrical tube which at first was open at both ends and shorter than 

 the length of the larva. At about the same time two lobes were differentiated at 

 the anterior end of the larva, one on each side of the median line. At a slightly 

 later period he thought he saw digitations of these lobes and he took them to be 

 the beginnings of the branchiae. 



Pagenstecher ('63) gives an account of the development of the branchiae and 

 operculum in Spirorbis. He states that the first traces of the branchiae are exhibited 

 in the form of three knobs upon each of the two head lobes. The operculum is not 

 differentiated until a later time when there is formed "der Fortsatz welcher ihn 

 tragen soil und der von den Tentakeln dutch eine Runzelung oder seichte Kerbung 

 der Oberflache ausgezeichnet war." Judging by Pagenstecher's figure there is 

 very little difference at the above-mentioned stage between the so-called opercular 

 outgrowth and the other branchiae. The figure gives two branchiae on one side and 

 two plus the opercular outgrowth on the other. 



Fritz Miiller ('64) noticed on the side of a glass vessel which he had on his study 

 table a young Serpulid with three pairs of branchiae, and which he took to be a 

 member of the Protula group because of the absence of an operculum. However, 

 a short time later he noticed that one of the branchiae had an opercular enlargement 

 at its end though it still retained its branchial pinnules. Still later the branchial 

 pinnules disappeared also and he had a Serpulid with the typical genus-Serpula- 

 type of operculum, which had developed by a modification of a branchia. In the 

 meantime a new pair of branchiae had been added to the oral crown making three 

 fbranchlae plus one operculum on one side and four branchiae on the other. This is 

 he only recorded observation of the transformation of a branchia into an operculum. 



