Chapter 2. 



The Sexual Organization of the Rhizocephala. 



In this chapter we will deal in detail with the sexual organization of our group, 

 concerning which many points have been left obscure by previous observers. The Rhizo- 

 cephala as a group are hermaphrodites, but unlike most hermaphrodites and unlike almost 

 all Cirripedes, they possess no means of cross-fertilization. Thus even in those species, such 

 as P. sulcatus, which occur gregariously on the same host, no organ is present for conveying 

 the spermatozoa from one individual to another, and this as a matter of fact never does 

 occur, while the majority of Rhizocephala are solitary, and cross-fertilization is impossible. 



On the other hand evidence has been produced by several authors, especially Fritz 

 Muller (5) and Delage (9), for the existence of complemental males such as are known to 

 exist among certain Pedunculata, and they have supposed that cross-fertilization, at any rate 

 during some stages of the animal's existence is effected by them, but this has never been 

 confirmed by actual observation. The interest of this question is considerable, owing to its 

 bearing on the principles which govern the evolution of sex, and especially the laws which 

 determine the self- or cross-fertilization of hermaphrodites; I need not therefore apologize 

 for going into considerable detail in describing the observations I have made on this subject. 



Before entering into this description I will shortly state the nature of the problem 

 before us, and indicate the solution at which I have arrived. Delage (9 p. 546 — 549) has 

 clearly brought out the fact that in Sacculina, at the same time as the internal epithelium of 

 the mantle secretes a chitinous investment, the vasa deferentia of the testes do the same, so 

 that they become blocked by a plug of chitin which is continuous with the chitinous invest- 

 ment of the mantle. When this is the case it is impossible for the spermatozoa in the testes 

 to escape into the mantle- cavity and to fertilize the eggs, but this is normally made possible 

 by the mantle and the vasa deferentia undergoing a moult immediately before the maturing 

 eggs are shed into the mantle-cavity, so that when this happens a free passage is left for the 

 spermatozoa to gain access to the eggs. But in Delage's opinion, this moult does not take 

 place previously to the expulsion of the first batch of eggs into the' mantle cavity, so that 



