TUNICATA. 89 



which pass from the bars of the branchial network to the outer wall 

 of the peribranchial chamber. In other cases, the number of gill- 

 slits is considerably reduced, and the gill is confined to the dorsal 

 part of the pharyngeal wall (Doliolum, Salpa). 



Generative Organs. The Tunicates are hermaphrodite ; the male 

 and female generative products, however, often attain maturity at 

 different times. The Salps especially, at the time of their birth, 

 have only the female organs, and it is not until later when they 

 are pregnant that the male organs attain maturity. In Perophora 

 the testes become mature first, in the Botryllidce the ova. The testes 

 and ovaries lie, as a rule, among the viscera in the hind part of the 

 body. The ovaries have the form of racemose glands, the testes of 

 blind tubes united in tufts. The generative ducts of both sexes 

 open into the cloacal chamber, in which (rarely in the place where 

 the germs originate) the fertilization of the ovum and the develop- 

 ment of the embryo takes place. The embryo either leaves the 

 cloacal chamber through the exhalent aperture while still enveloped 

 by the egg-membranes, or is nourished by a sort of placenta and 

 born at a more advanced stage of development (Salpa). 



In addition to the sexual reproduction, the asexual reproduction 

 by means of budding is very general, and frequently leads to the 

 formation of colonies with very characteristic grouping of the indi- 

 viduals. The budding sometimes takes place on different parts of 

 the body, sometimes is confined to definite places or to a germ-stock 

 (stolo prolifer). The colonies thus produced do not by any means 

 always remain fixed ; but, as e.g., Pyrosoma, may possess the power of 

 moving from one place to another, or, as in the Salp-chains, they can 

 swim tolerably rapidly. 



The embryonic development of the Ascidians presents a great re- 

 semblance to that of the lower Vertebrates, a.nd more especially to that 

 of Amphioxus. After the completion of the total segmentation a two- 

 layered gastrula is formed, from the ectoderm of which the neural tube 

 is developed. At the same time an axial skeletal structure, like the 

 chorda dorsalis, arises from a double row of endoderm cells. The 

 relative positions of the alimentary canal, the nervous system and the 

 notochord are analogous to those of the Vertebrates. 



The post- embryonic development of the Ascidians is complicated. 

 The embryos leave the egg-membranes as movable larvae (Ascidian 

 tadpoles) provided with a swimming organ (tail) and an eye-spot. 

 They swim about freely for some time, and in many cases produce a 

 small colony by budding before becoming fixed. In the Salps and 



