412 



PHYSIOLOGY OF THE INVERTEBRATA. 



The embryo arises from the fecundated ovum without any 

 metamorjjhosis. The ovum (Fig. ^6, /) undergoes segmenta- 

 tion within the ovary, and the embryo is born alive as a 

 ciliated larva, possessing a somatic or digestive cavity and a 

 mouth. After this two mesenteric tissues are produced which 

 divide the internal chamber into two unequal parts. Two 

 new mesenteries subsequently arise in the larger or anterior 









j^vyTv.^^*^ 



^li.'<>^^- 





Fig. 76. — Spermatozoa and Ova of certain Invertebrates. 



a = Lumbricus. fi = Pagurus. f — Pisa. d = Grapsus. e = Astacus ((? )• 

 / — Actinia. ^ = Astacus (?). 



chamber. A third pair are next develojjed in the posterior 

 chamber, and then a fourth jDair in the lateral spaces. Next 

 the tentacula are developed ; and afterwards four new mesen- 

 teries appear, these are situated one on either side of the two 

 primary mesenteries, so that in all twelve somatic cavities are 

 formed which ultimately become of equal size. 



The Actincv are also reproduced by gemmation or budding. 



For fuller details the reader is referred to special books 

 and memoirs on the subject.* 



The ECHINODERMATi. 



These animals propagate by sexual organs, and the sexes 

 are distinct; hermaphroditism is very rare. The ova are 



* See also Prof. A. Giard's paper in Comptes Hendus de VAcadimie des 

 /Sciences, 1877. 



