310 



SEXUAL DIMORPHISM 



mouth. In the female the ova undergo the earliest stages 

 of development in small pockets formed on the external 

 surface of the arms, so that here again the modification is 

 associated with the special stimulation due to the presence 

 of the ova and larvae, and therefore not applied to the arms 

 of the male. 



In the Hydrozoa generally, there is a very marked poly- 

 morphism associated in many cases with reproduction, though 

 different in its relations from sexual dimorphism. I refer to 

 that difference of the forms of individuals which constitutes 

 what is generally called the alternation of generations. The 

 usual history is that the egg gives rise to a polyp form having 

 a cylindrical body with a terminal mouth surrounded with 

 tentacles. This form of zooid does not usually produce ova 

 but proliferates by buds, which grow into similar zooids 

 connected with the original stock, so that a branching plant- 

 like compound structure is produced. This hydroid stock is 

 attached to the sea-bottom. When it is mature, however, 

 some of the zooids produced develop into the Medusa or 

 jelly-fish form, which become detached and swim away inde- 

 pendently. The Medusas produce eggs and sperms, and are 

 of separate sexes. It is difficult to decide what was the 

 original form of the zooid in these cases, whether the Medusa 

 or the polyp, but it is at least obvious that the difference of 

 structure corresponds to the difference of the conditions of 

 life. The characteristic organ of the Medusa is the bell- 

 shaped expansion, which is essentially a swimming organ, 

 and constantly performs muscular contractions. Here again, 

 therefore, we find the general principal holds, that structural 

 evolution is controlled by functional activities dependent on 

 the stimulations involved in the conditions of life. 



