4 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



immediate ancestor of the Cirripedia, we must still bear in mind that 

 the development is not necessarily a recapitulation of the phylogeny. 

 This point was overlooked by Fritz Miiller in his classical work " For 

 Darwin." This philosopher, seeing' that the typical development of 

 the Crustacea was by means of a Nauplius, concluded that the Nau- 

 plius represented an ancestral form of the group. To make this clear 

 we may suppose that we have two Cirripedes, A x Bi Ci Di and 

 A 2 B 2 C2 D2 : the close resemblance of the stage Ai to A 2 , of Bi to 

 B 2) &c., may at first sight seem only explainable on the view that 

 A, B, and C represent ancestors. Thus if B x and B 2 represent Nauplii 

 of two species Di and D 2 of the group, the resemblance between 

 Bi and B 2 is so striking (extending even to the minute character of 

 the numerous hairs and other processes shown on the appendages) 

 that we can only conclude that similar characters were possessed by 

 the ancestral form ; one might then jump to the conclusion that the 

 Nauplius must represent an ancestor of the group : such a con- 

 clusion is now known to be unjustified, for the characters, though 

 necessarily present in the ancestor at some stage in the development, 

 were by no means certainly found in the adult condition ; in fact 

 so far as we can tell from the development of the two types, it is 

 clear that they did not belong to the adult of the immediate ancestor, 

 for the two adults present an equally great resemblance to one 

 another in a totally different way, comparatively few of the striking 

 characters of the Nauplius being found at this stage : the resem- 

 blances in the case of the adults are of such a character that we can 

 only conclude that the similar characters in their case also, are due to 

 inheritance from a common ancestor : the same may be said of each 

 of the other stages, and it clearly follows that all we can conclude 

 from the similar development of the two forms Ai B x Ci Di and 

 A 2 B 2 C 2 D 2 is that the common ancestor had a stage corresponding 

 to each of foregoing stages, and that the development may be repre- 

 sented by the formula A B C D. It does not, then, follow from this 

 alone that B, for instance, is an ancestor of Di or D 2 ; at the same 

 time nothing disproves this view : the question must be decided upon 

 other evidence. 



If the modification of forms leading to D has been a progressive 

 one, C will represent an ancestral form, and its life-history will be 

 ABC: the discovery of such a form would appear to furnish the 

 only satisfactory evidence as to the ancestral history of D. Judged 

 by this standard, the trochosphere or larva of many annelids, mol- 

 luscs, &c, has great value, for we find a number of adult animals 

 which are either practically in the trochosphere condition (Rotifera, 

 Polyzoa, &c.) or are but little removed from it. The same inay.be 

 said of the embryo of Birds and Mammals with its gill-slits, for we 



