394 COMPAEATIVE ANATOMY. 



organ for the enclosed parts. The consistency of this covering varies 

 from the softness of jelly to the hardness of cartilage. It is 

 generally of glass-like transparency, and in the Ascidians often 

 variously coloured. Complications of the structure of the mantle 

 are brought about in many Ascidians (Phallusia) by the penetration 

 into it of great numbers of blood-vessels. Very remarkable is the 

 development of the mantle into two valves, movable on one another 

 like the shells of the Lamellibranchiata, and which are able to open 

 and shut as do the latter (Ohevreulius). In the colony-forming 

 kinds, this mantle-layer is common to all the persons, enclosing 

 them together. 



The formation of this mantle leads to a suppression of the 

 differentiation of all other tegumentary organs ; we have never- 

 theless to notice in the Copelata the formation of numerous other 

 organs of this category — unicellular glands, hair-like processes, etc. 



A mass of cells is also developed from the ectoderm in the 

 Pyrosomas, and occupies a position in the neighbourhood of the in- 

 current orifice ; it is the paired luminiferous organ of these animals. 



Hertwig, 0., Ueber den Bau und die Entwickelnng des Timicateimiantels. 

 Jeuaische Zeitschr. Bd. VII. 



Skeleton, 



§ 303. 



In most Tunicata, the mantle, in virtue of its rigidity, functions 

 as the supporting organ of the otherwise soft body. Besides this 

 "We meet with a peculiar organ of the greatest morphological import- 

 ance. In the tail-like propeller of the Appendicularia? there exists 

 an axial organ which extends forward as far as the fore-body. It is 

 made up of cells, which secrete a fairly resistent chord consisting of 

 homogeneous substance enclosed in a continuous sheath, the remains 

 of the cells being found at a later period still resting upon the chord. 

 This chord acts by means of its elasticity, bringing the propelling- 

 tail back to its previous position after it has been bent by the con- 

 traction of its muscles. Such an axial organ (Fig. 208, ch) is retained 

 by all those Tunicata-larvae which possess a mobile propelling tail, 

 both Ascidians and Cyclomyaria. It disappears with the tail. Its 

 relations of position show it to be homologous with the chorda 

 dorsalis of the Vertebrata, and we may therefore designate this 

 structure also as chorda. 



Muscular System. 



§ 304. 



The disposition of the musculature presents very different 

 features in the various Tunicata. The Copelata, for instance, 



