110 GENERAL PRINCIPLES OF ZOOLOGY 



todes) there are glands (vifellaria), distinct from the ovaries, which form the 

 yolk cells (fig. 75). 



Secondary Sexual Characters. Often we can distinguish between the 

 male and female of dioecious organisms only by the sexual products (medusas, 

 polyps, sponges). In other cases the sexual ducts are also characteristic. In 

 the higher animals these primary sexual characters are associated with those of 

 a secondary nature so that it is possible to recognize male or female at a glance. 

 These secondary sexual characters are exemplified in many birds and mammals 

 by the voice, the hair or feathers, strength of muscles and skeleton, presence of 

 offensive or defensive weapons, etc.; in insects by structure and markings of 

 wings, form of antennae, etc. (fig. 74). This sexual dimorphism may become 

 so marked that only careful study, especially of the development, shows that the 

 male and female belong to the same species; dwarf males of Bonellia (fig. 268), 

 Cirripedia, Copepoda (fig. 8). 



A part of these secondary sexual characters are developed at the approach of 

 sexual maturity and can be restricted or even suppressed when the gonads are 

 destroyed or removed (castration). This leads to the conclusion that the 

 development of the secondary sexual characters is correlated with the matura- 

 tion of the gonads and is influenced by it. As a causal factor it is thought that 

 'internal secretions' (hormones) arise in the sex glands; these are passed into the 

 circulation and cause the modification of distant parts like hair, larynx, mam- 

 mary glands. 



Yet this explanation must not be carried too far. Many secondary sexual 

 characters, like those connected with the genital ducts, develop independently 

 of the gonads. The peculiar developmental direction taken by the dwarf males 

 just alluded to is begun before the maturation of the testes and apparently 

 would appear even if the anlage of the gonads were removed in the embryo. 

 We are on firmer ground with the corresponding modifications in the Lepidop- 

 tera. Here the secondary sexual characters clearly develop in the way laid 

 down in the embryo, if the gonads be removed from the young larva; even if 

 the testes are removed and replaced by ovaries taken from other individuals, or 

 vice versa. The transplanted gonads become mature, while the rest of the 

 sexual apparatus and the secondary sexual characters show the peculiarities of 

 the original sex. All of these observations show that a correlation of gonads, 

 genital ducts and secondary sexual characters exists to only a limited extent. 

 The harmonious development of parts is rather regulated by a third factor, 

 the peculiarities of the fertilized egg or its early developmental stages. It is 

 these that prescribe, in a more or less striking manner, a certain developmental 

 direction, not only for the gonads, but for the whole organism. 



Animal Organs. 

 I. Organs of Locomotion. 



Voluntary Locomotion. The power to change their location volun- 

 tarily is a peculiarity so prominent in animals that usually it is sufficient 

 for deciding whether an organism belongs to the vegetable or to the 

 animal kingdom. On this account it is necessary to call attention to the 

 fact that numerous animals, freely mobile in the larval stages, lose the 

 power of locomotion, becoming fixed to the ground, to plants, or to other 

 animals, and only retain the power to move parts of the body, as in the 

 corals the crown of tentacles, the barnacles their feathery feet; many 

 attached molluscs can actively close the shell. 



