THE OESTOIDEA. 



185 



After a time, a cavity appears in the midst of the cells of 

 which the morula is composed, and a chitinous cuticula is 

 developed upon the outer surface of the embryo. Ramified 

 water-vessels make their appearance in the wall of the sphe- 

 roidal sac thus formed, and in some cases open by an external 

 pore. There is, therefore, a very close resemblance between 

 this cestoid embryo and the sporocyst of a Trematode. 



When the saccular embryo has attained a certain size, a 

 thickening and invagination take place, usually at one (7'ce- 

 nia), sometimes at many (Coenurus, JEchinococcus) points of 

 its wall. The invagination of the wall elongates inward, and 

 becomes a caecum, the cavity of which opens outward. At 

 the bottom of the interior of this caecum, and therefore on 

 what is morphologically its external surface, the hooks of 

 those species which possess them are developed, while, upon 



I 



% #4.-' HW 



Fig. 4%.—Echinococcu8 veterinarian.— A, "Taenia head," or Scolex: a, hooks; 6, 

 suckers; c, cilia in water-vessels; d, oval, strongly refracting particles; B, single 

 hooks; C, portion of the elastic cyst, a ; with the inner membranous primary 

 cyst, b; c and e, Scollces developing from its inner surface; tf, a secondary cyst. 



the side-walls, elevations arise, which become converted into 

 suckers. The caecum is next evaginated or turned inside 



