52 Traces of Unity in the Appendiciilar 



for the evidence upon which it is possible to come to a 

 conclusion upon this point, and upon that which was 

 last under consideration, is not yet available without 

 going very much out of the way to find it. 



Nor are the traces of a common plan absent in 

 that case which now alone remains to be noticed 

 namely, that of the external genital appendages. In 

 the male prawn, on the under surfaces of the basal joint 

 of the last ambulatory leg, is an opening the edge of 

 which may be everted so as to form an intromittent 

 organ ; in the female, in the corresponding part of the 

 third ambulatory leg is an opening into which the 

 intromittent organ of the male is inserted at the proper 

 time. This is all. The case recalls the presence of the 

 sexual openings at the base of the rays of the starfishes, 

 which are either male or female, or both ; nay, this 

 difference between male and female openings is not so 

 great as it seems to be at first, even in the crustaceans, 

 for in the prawn, and lobster and many of their con- 

 geners it not unfrequently happens that male openings 

 are developed on one side of the body and female 

 openings on the other. Nor are these the only proofs 

 of the close relationship between the organs in question 

 and the limbs of which so much has been said. In the 

 male spider, for example, the tumid and unarmed 

 terminations of the long, maxillary palps, which palps 

 are legs to all intents and purposes, contain the parts 

 analogous to vesiculae seminales, and are in reality 

 limbs modified and devoted to a sexual purpose. And 

 again, the spermatophorous or sexual arms of the 

 octopus, or the " sail " of the argonaut, may be looked 

 upon in the same light, for it is now known that each of 

 these parts agrees with the ends of the maxillary palps of 

 the male spider in containing what must be regarded as 



