NEEYOUS SYSTEM OF TUNtCATA. 



395 



possess a complete muscular layer with longitudinal fibres, on the 

 propulsory tail only. It can be divided into a dorsal and a ventral 

 band of varying breadth, covering in the chorda from above and 

 below. Muscles are entirely absent from the anterior division of the 

 body which contains the viscera. 



In the Ascidians the musculature forms a sac lying beneath the 

 ectoderm, and is separable in Cynthia into several layers according 

 to the course of the fibres. In others the muscular layer is feebler, 

 composed of intercrossing bands (A. composite) . The Pyrosouue 

 have no muscles excepting around the excurrent and incurrent 

 orifices of the body. The musculature of the Cyclomyaria is broken 

 up into isolated annular bands ; and in the Salpae also it forms hoops, 

 which are, however, here and there joined to one another. This 

 hoop -like formation arises from the differentiation of a primitively 

 continuous muscular layer. Gaps arising in this become gradually 

 larger until the breaking up of the layer into separate hoops is 

 brought about. At the incurrent and excurrent opening of the 

 Ascidians the musculature has also an annular disposition and has 

 the characters of a sphincter. 



The form-elements of the musculature are transversely striated. 



Nervous System, 



§ 305. 



The central apparatus of this system occupies a dorsal position 

 in all Tunicata, and proceeds from a differentiation of the ectoderm, 

 as has been ascertained from the study of the development of 

 Ascidae and of Salpa3. In the general relations of the parts there are 



Tig. 208. Ascidian embryo with only a part of the tail C. N Nerve-centre, forming 



a cavity in front N', produced behind into a nerve-cord n. Eye. a Auditory organ. 



K Embryonic foundation of the respiratory chamber, d Ditto of the digestive canal. 



o Ditto of the mouth, ch Chorda (after Kupffer). 



agreements with what we find in the lower Worms. The in-sinking 

 of the ectoderm forms a tube which remains open for a time, and 

 then becomes pinched off from the surface-layer and extends itself 

 in Ascidian larvas (Molgula) as a chord reaching throughout the length 

 of the tail (Fig. 208, to). A central canal perforates the chord, and 



