. Elements of Comparative Anatomy. NEEVOUS SYSTEM OF TUOTCATA. 395 possess a complete muscular layei' with, lougitudinal fibres, on tlie 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 Ascidiaus 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 ban
. Elements of Comparative Anatomy. NEEVOUS SYSTEM OF TUOTCATA. 395 possess a complete muscular layei' with, lougitudinal fibres, on tlie 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 Ascidiaus 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. compositee). The Pyrosoma3 have no muscles excepting around the excurrent and incurreut orifices of the body. The musculature of the Cyclomyaria is broken up into isolated annular bands; and in the Salpte 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 incurreut and excurrent opening of the Ascidians the musculature has also an annular disposition and has the characters of a sphincter. The form-elements .pf 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 Ascidse and of Salpte. In the general relations of the parts there are. Fig. 208. Ascidian embryo with only a part of the tail G. N Nerve-centre, forming a cavity in front N', produced behind into a nerve-cord n. 0 Eye. a Auditory organ. K Embryonic foundation of the respiratory chamber, d Ditto of the digestive canal. 0 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
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