Quarterly Journal of Microscopical Science, Vol s2-81, 127-150, Copyright © 1938 by Company of Biologists

Memoirs: Abdominal appendages in the female and copulatory appendages in the male Asellus

1. In the female Asellus aquaticus it is the first pair of pleopods which is missing and not the second pair, as was usually held. This condition is the result of an inhibition on the development of the last thoracic and first abdominal segments in later brood-pouch stages, an inhibition which is only temporary in the last thoracic segment and in the first abdominal segment of the male.

2. The resemblance between the second abdominal appendages in the female and the first pair of the male is therefore purely convergent.

3. The strong probability that the same condition holds throughout the sub-order Asellota is supported bj a comparison of adult morphology, and should be verified by embryological study. It is more usual in Crustacea for appendages to be lost at the end than in the middle of a series.

4. The structure of the copulatory apparatus of the adult male Asellus is extremely complex, and apparently closely adapted to its mode of function. In contrast the female apparatus is of the simplest.

5. Points of evolutionary and genetical interest are raised by the condition of these appendages in the two sexes.