chap, v.] HOMCEOSIS IN ARTHROPOD A. 149 



hind wing but its colour was rather yellower, though it was more 

 red than yellow. I have to thank Mr Richardson for allowing me 

 to examine this specimen in company with Dr Sharp. In compliance 

 with Mr Richardson's wish we did not strip the wing or remove the 

 thick hairs which surrounded its base, and it is therefore not 

 possible to speak with certainty as to its precise point of origin. 

 The following description of it was drawn up for me by Dr Sharp : 

 " The supernumerary wing projects on the under side of the body, 

 and at its base there intervenes a space between it and the dorsal 

 region of the body about equal to the length of the metathoracic 

 side-piece. The exact attachment of the base of the supernumerary 

 wing cannot be seen owing to the hairiness of the body, but so far 

 as can be seen it is to be inferred that the wing is attached along 

 the length of the posterior coxa, the outer edge of the point of 

 attachment may be inferred to extend as far as the suture between 

 the coxa and thoracic side-piece ; if this view be correct the 

 abnormality may be described as the absence of the hind femur 

 and parts attached to it, and the addition of a reduced wing to the 

 hind-margin of the coxa. It is, however, just possible that if the 

 parts could be clearly distinguished it might be found that the 

 real point of attachment of the abnormal wing is the suture 

 between the metathoracic side-piece and the hind coxa." 



It should be distinctly stated that there is no empty socket or 

 other suggestion that the rest of the leg had been lost, and it was in 

 fact practically certain that it had never been present. There is 

 thus a strong prima facie case for the view that the leg has been 

 developed as a wiug, however strong may be the theoretical 

 objections to this conclusion. On the other hand, as will be shewn 

 in a later chapter, supernumerary wings are known in specimens 

 having a full complement of legs, and it is conceivable that one of 

 these supernumerary wings may have arisen in such a way as to 

 prevent the proper development of the leg from the imaginal disc. 

 If the specimen were carefully stripped of hairs some light might 

 perhaps be thrown on this question. The figure (Fig. 17) is from 

 a drawing kindly lent me by Mr Richardson. 



Crustacea. 



Cancer pagurus. Specimen having the right third maxilli- 

 pede developed as a chela. This animal was brought by a fisher- 

 man to the Laboratory of the Marine Biological Association at 

 Plymouth. It is a male, measuring five inches from one side of 

 the carapace to the other. All the parts appear to be normal with 

 the exception of the third maxillipede of the right side. This 

 structure, however, has the form shewn in Fig. 18, A, differing 

 entirely from the ordinary condition of the appendage. Fig. 18, B, 

 is taken from the third maxillipede of the left side and shews the 

 ordinary structure of the same parts. On comparing the two 

 figures it will be seen that the protopodite does not differ in the 



