144 GENERAL PRINCIPLES OF ZOOLOGY. 



provided with sac-like evaginations which serve for the re- 

 ception of the semen. In the oviduct, these are called rc- 

 ccptacula scminis, in the vas deferens vesiculce seminales ; the 

 former give lodgment to semen which has entered the 

 female sexual passages during coition, the latter to semen 

 which has been formed in the true testes. 



Accessory Sexual Apparatus. The terminal portion 

 of the vas deferens is often very muscular and is called the 

 ductns ejaculatorius ; it may be evaginated as a penis or 

 cirrus, and then projects beyond the surface of the body. 

 The terminal portion of the oviduct is usually widened so 

 that two portions may be distinguished, the uterus, which 

 harbors the eggs during their development, and the vagina, 

 which serves for copulation. In addition there may occur 

 in both sexes other accessory glands of the most diverse 

 character. 



Animal Organs. 

 I. Organs of Locomotion. 



Voluntary Locomotion. The power to change their 

 location voluntarily is a peculiarity so prominent in ani- 

 mals that to the "laity' it is sufficient for determining 

 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 lose the power 

 of free locomotion, becoming fixed to the ground, to plants, 

 or to other animals. All sponges and corals, most hydroid 

 polyps, and the crinoids among the ccliinoderms, are fixed, 

 and have thereby obtained such a conspicuous similarity to 

 plants that, although they are true animals, they were long 

 regarded as plants. Further, many mussels and worms are 

 firmly fixed within their cases; indeed, many crustacean 

 forms, the cirripeds, have completely lost their free motil- 

 ity. But a more careful investigation in all these cases will 

 show that the power of moving the separate parts exists, 

 for the corals can draw in their crown of tentacles, the 



