142 



ESSENTIALS OF ZOOLOGY 



Reproductive Organs. The sexes are separate. The 

 ovaries, as well as the testes, are about twenty-six pairs, 

 developed from diverticula of the myocoels of the pharyngeal 

 region. When ripe the products escape from the gonads, in 

 each case by an opening formed specially, then into the atrial 

 cavity. They are carried to the exterior by the atriopore, 

 and fertilisation takes place externally. 



Soknocyte 



Efferent branchial 

 blood-vessels 



Oittbar 



Protoneph. epithelium 



Coelomic epithelium 



^^^^^^^^ 



FlageUa 



Atrial ectoderm 

 FIG. 67. Amphioxus. Section of a protonephridium. After Goodrich. 



Development. 1 The eggs and the early and larval stages 

 of development are planktonic. There is during the long 

 period of at least three months a wide dispersion by passive 

 drift or denatation. This is the reason that the adults are 

 found in situations which are often widely separated. It 

 has been proved, moreover, that the larvae may be transported 

 across the ocean. 



The ripe ovum of Amphioxus measures about O'l mm. 

 It is covered by a fine structureless membrane, and the outer 

 layer of protoplasm is differentiated to form an additional 



1 1881. Hatschek, Stud. u. Entwicklung des Amphioxus. 



