286 



ZOOLOGY 



SECT. 



the duct of a large spermatlicca (spth.) or chamber for storing the 

 sperms received in copulation. In the male Gordius the testes 

 are not known: they seem to disappear very early, after discfiarg- 



spth 



FIG. 232. Gordius. A, horizontal section of female, showing ovaries (0-77) attached to mesen- 

 tery (in -x.) ; /'. >r. body-wall ; B,.. posterior extremity of male, sagittal section. I. <:. bursa 

 copulatrix ; cl. cloaca ; int. intestine ; t. tail ; p. . <</. ventral nerve cord ; <.<. sem. vesicula 

 seminalis ; C, posterior extremity of female, sagittal section. (//(/>. gonopore ; .~<j>t/t.. sperma- 

 theca ; at. viterns ; cay. vagina ; <. ,ic. c<L ventral nerve cord. (After Vejdowsky.) 



ing their contents into large reservoirs or vcsicidce scminales (B, 

 vs.sem^ : from these vasa deferentia are continued into a cloaca (cl.) 

 or dilated extremity of the intestine, part of which can be everted 

 as a bursa copulatrix (b.c.). 



The development of Nematoda has been best worked out in 

 Ascaris nigrovenosa. Segmentation is complete, but somewhat 

 unequal (Fig. 233, A, B), and the ectoderm cells grow over the 

 endoderm (C), forming an epibolic gastrula with a long slit-like 

 blastopore (D-F). The mesoderm is developed from a single pair 

 of endoderm cells (D., mes.), which enlarge, multiply, and form a 

 distinct cell-layer between ecto- and endoderm (E-H). TJie epithe- 

 lium of the pharynx is formed by an invagination of ectoderm, 

 so that this division of the enteric canal is a true stomodseum 

 (I, stdm.). . The nervous system is developed from certain cells 

 (G-I, n.) which bud off from the ectoderm at the anterior end of 

 the body. The reproductive organs originate from a single meso- 

 derm cell on each side (H, I, #.). 



Many of the Nematoda have a curious and complex life- 

 history : a few examples will be selected for description. 



Ascaris nigrovenosa lives, in the sexual condition, in the lungs 

 of Frogs and Toads : it is remarkable among members of the class 



