234 



ZOOLOGY 



or to the exterior. It is believed that in some instances at least 

 the genital ducts are modified nephridia. 



It will be a profitable practical exercise for the student to 

 make for himself a diagram of the sex organs of the earthworm 

 or some other hermaphrodite form by reference to several stand- 

 ard texts. 



273. Reproduction and Development. — Sexual reproduction 

 appears in all groups. As we have seen, copulation may occur 



Fig. 105. 



mi.-' 



Fig. 105. Two stages in the development of Nereis. A, 8-celled stage; B, i6-celled stage, 

 both viewed from the active or ectodermal pole, mi.^, mi.\ and mi.\ the first, second and third 

 sets of micromeres separated from mo., the macromeres; i^ first somatoblast. one of the second 

 group of four cells to be budded from the macromeres; s^, second somatoblast, one of the third group, 

 which gives rise to the mesoderm. The micromeres are ectodermal and the macromeres produce 

 the entoderm. (After Westinghausen.) , 



or the elements may come together in the water. In the Oligo- 

 chseta and leeches the fertilized ova, or the ova together with 

 masses of spermatozoa, are enclosed in a cocoon of secreted 

 material and within this case the young worm is developed. In 

 the Polychasta the larva undergoes its development in a free 

 state. Segmentation in Annelida is complete and usually un- 

 equal, giving rise at the eight-celled stage to four micromeres and 

 four macromeres (Fig. 105). The micromeres produce the ecto- 

 derm; directly or indirectly the macromeres give rise to the 

 entoderm. Early in the cleavage "primitive mesoblasts" — 

 cells which produce the mesodermal structures — are separated 

 from the macromeres. A gastrula is formed either by invagina- 

 tion or by overgrowth. In the earthworm (Oligochseta) the 

 blastopore of the gastrula forms the mouth of the adult worm.. 



