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Quarterly Journal of Microscopical Science, Vol s2-83, 61-89, Copyright © 1941 by Company of Biologists
1 Department of Zoology, Bedford College, University of London
1. The anterior abdominal appendages of A. meridianus, while showing fundamental similarity to those of A. aquaticus, show considerable differences in structure, function, and development.
2. Neither species shows a complete superiority over the other in respect of these differences, nor is there as yet reliable evidence from experiment or from observations on distribution as to their relative viability.
3. The evidence from development in sum indicates that the A. meridianus group is the more primitive, the A. aquaticus type recapitulating an A. meridianus stage. Both groups have, however, evolved considerably since their first divergence, the changes affecting all stages of ontogeny; in respect of one species difference in particular, A. meridianus shows overstepping of the A. aquaticus condition which is primitive.
4. In the male A. meridianus there is considerable functional transposition between parts of the system as compared with A. aquaticus.
5. The developmental studies indicate further: (1) the extreme regularity of the processes, even to the smallest detail; (2) the independence of growth and development on periodicity in moulting; (3) methods of addition of setae during development.
6. In the light of these studies the female intersexes of Maercks (1931) are seen to correspond to stages in normal male ontogeny, but with asynchronizations between parts of the appendages.
7. The intersex of Unwin (1920) may have been a species hybrid. He was working with A. meridianus, rather than with A. aquaticus as he believed.
8. A case of compound heteromorphosis affecting the anterior abdominal appendages of A. meridianus is described, and the importance of the peripheral motor nerves in the determination of the phenomenon is indicated.
9. Eegeneration in these appendages may be quite normal, when it shows an accelerated recapitulation of ontogeny. It may be abnormal in showing lack of synchronization between parts, or even complete suppression of some. The rate of regeneration increases with age up to a point, and then decreases. It also decreases with the amount of an appendage removed.
10. Relative independence and lack of synchronization between parts is thus seen in intersexes, in other abnormalities, and in regeneration, and in all cases may depend on peripheral motor nerves.
11. Chains of sense cells and hollow chitinous ingrowths associated with them are attached to the base of many of the setae in Asellus.
