PHYSIOLOGY OF THE INVERTEBRATA. 419 



or ova according to the sex of the animal containing them. 

 The genital organs are situated in the lateral part of the 

 body between the pouch-like diverticula of the intestine. 

 They are arranged in pairs along the body and open externally 

 by paired apertures. The development of Nemertcs occurs 

 with metamorphosis. The following are the stages through 

 which the embryo passes : («) Before hatching — eg^, morula, 

 and planula. (h) After hatching — a ciliated larva or Pilidiitin 

 is formed ; and the adult condition is attained by direct 

 growth, or by budding out from the FUidium. 



In the liotifcra the sexes are distinct, and the male animals 

 are much smaller than the females. The genital apparatus 

 of the male consists of a testis, in which the spermatozoa are 

 produced and stored ; the testis opens by a duct situated 

 near the posterior end of the body, usually on a muscular 

 protruberance or penis. The male llotifera are short-lived, 

 and are only born into the world to impregnate the ova of 

 the females. The genital apparatus of the female consists 

 of a round sac-like ovarium or ovary, filled with ova in 

 various stages of development, and a short oviduct which 

 opens into the cloaca. The female produces both summer 

 and winter eggs. According to Prof. Huxley the winter eggs 

 are produced partheuogenetically — i.e., without previous im- 

 pregnation. In fact, he says in regard to Lacinnlaria that the 

 winter eggs appear to be " segregated portions of the ovarium." 

 On the other hand, Cohn believes that it is the summer eggs 

 which are produced partheuogenetically, while the winter eggs 

 are impregnated. The egg undergoes complete and irregular 

 segmentation ; then a two-layered embryo is formed. An 

 involution of the epiblast occurs, giving rise to the primitive 

 mouth, which remains permanently open. The trochal disc 

 grows out of the walls surrounding the epiblastic depression ; 

 and the nerve-ganglion is also produced from the epiblast. 

 At the bottom of the primitive depression, the true mouth is 

 formed ; while the oesophagus and the remaining portion of 

 the alimentary canal are developed from the hypoblast. 



