chap, v.] HOMCEOSIS IN ARTHROPOD A . 155 



1. That this Variation may be bilaterally symmetrical, 

 but that the evidence goes to shew that it is more often uni- 

 lateral. 



2. That there is a clear succession between the several 

 oviducal openings, those of the antepenultimate legs being 

 the largest, the penultimate the next, and those of the last 

 legs the smallest. 



3. That Homceosis may occur between segments which 

 are not adjacent, as in the case of extra oviducal openings on 

 the last thoracic legs, none being formed on the penultimate 

 (No. 84). 



4. That the Variation may be perfect. 



With the foregoing, the following evidence may be compared, 

 though it is very doubtful whether it properly belongs here 1 . 



86. Cheraps preissii [an Australian freshwater Crayfish, nearly allied 

 to Astacusj. Of seven specimens received one was a normal male 

 and three were normal females. The other three had on the basal 

 joint of the third [antepenultimate] pair of legs a round opening, 

 having the size and shape and situation of the normal female openings. 

 These apertures were closed with soft substance. The fifth legs bore 

 the usual male openings, from which the ends of the ductus ejacula- 

 torius protruded. The coiled spermatic ducts were normal ; but no 

 ovary was found and no internal structure was connected with these 

 female openings, von Martens, E., Sitzb. Ges. naturf. Fr. Berlhi, 1870, 

 p. 1. 



87. Astacus pilimanus J, a single specimen, and A. braziliensis J, 

 a specimen collected by Hensel in Southern Brazil, a similar opening 

 was found on the third pair of legs ; but in other specimens of these 

 forms there was only a slight though sharply defined depression in 

 the chitinous covering at this point, von Martens, E., I.e. 



1 See also Nicholls, ~R.,Phil Trans., 1730, xsxvi. p. 290, figs. 3 and 4 describing 

 a Lobster {Homarus vulgaris) having male organs on the left side and female organs 

 on the right. 



