PHYSIOLOGY OF THE INVERTEBRATA. 429 



secretions of the alimentary canal, and the embryos becoming 

 free, bore their way through the walls of the intestine of 

 their host. While in the alimentary canal of the host, the 

 embryo loses its spines, and develops into an elongated larva. 

 In this condition, the skin gives rise to the muscular body 

 wall, the dermal vessels, and the lemnisci of the adult ; all 

 the other organs are developed from the " embryonic nucleus," 

 which makes its appearance early during the embryonic 

 development — i.e., before the ovum is " swallowed " by one of 

 the above-mentioned Invertebrates. Pin^lly, the embryo finds 

 its way into the alimentary canal of one of the Vertebrata 

 (e.g., fishes, aquatic birds, pigs, &c.), and while there, it 

 develops sexual organs. 



The CHiETOGNATHA. 



This group contains only one genus — Sagitta. These 

 animals are hermaphrodites ; and the male and female 

 organs are situated at the sides of the posterior end of the 

 body. There are two tubular ovaries, the ducts of which 

 open externally on each side of the anus. The tubular testes, 

 situated behind the ovaries in the caudal region of the body, 

 open by short ducts at the sides of the tail. The fecundated 

 ovum becomes completely segmented, giving rise to a blasto- 

 sphere. By invagination, the hemispherical, two-layered, cup- 

 shaped gastrula is formed. The primitive opening now closes, 

 and a permanent mouth is formed at the opposite end. At 

 this point the embryo has an oval shape, but it finally elon- 

 gates and acquires the adult form before leaving the egg. 



The Onychophora. 



Professor H. N. Moseley* has shown that in Peripatus the 

 sexes are distinct. The testes are egg-shaped ; and they are 

 provided with coiled vasa deferentia, which ultimately unite, 

 forming a common duct. This duct opens on the ventral 



* Philosophical Transactions of the Eoyal Society, 1874. 



