390 COMPAEATiVE AN"ATOMY. 



Form of the Body. 



§ 300. 



The form of the body of the Tunicata undergoes in the various 

 divisions such remarkable modifications, that the resulting forms 

 compared in their extreme conditions of development appear to 

 have no relationship whatever to one another. In the Copelata, the 

 lowest Tunicata at present known, the body exhibits two divisions, 

 of which one encloses the most important organs, whilst the other 

 has the form of a very wide and long movable appendix — the pro- 

 pelling tail. The anterior division of the body contains the intestinal 

 tract together with its respiratory portion. The gut opens to the 

 exterior by the ventrally-placed anus. Two slits break through the 

 body-wall from the respiratory cavity. On the whole a bilateral 

 symmetry predominates, and accordingly two antimeres are recog- 

 nisable. The tail portion of the body in the Copelata, attached 

 ventrally to the fore-portion, projects at an angle from the latter, 

 and thus has the appearance of a simple appendage. How the earlier 

 stages explain this condition must be described below. 



A form coming near to this condition is possessed by the larvse 

 of the Ascidians, in which the tail is simply a prolongation of the 

 aboral end of the body, and so far appears to present us with a more 

 primitive condition. A similar process is found also in the young of 

 Doliolum, whence it may be supposed that all Tunicata have descended 

 from a common ancestor provided with such a tail-like region of 

 the body. In Doliolum, as the power of swimming becomes associ- 

 ated with modifications of the respiratory cavity, the propelling tail 

 dwindles. In the Ascidians free locomotion is lost when the pro- 

 pelling organ atrophies. The complete animal takes on a fixed 

 mode of life. Whilst a greater complication of the structure of the 

 organism is acquired the external appearance becomes simplified. 

 The tube-like body exhibits two openings, approximated to one 

 another. The orifice of entrance corresponds to that of the Copelata. 

 The second opening leads into a chamber having the function of a 

 cloaca, which has arisen from a development in connection with the 

 primitive respiratory slits. This condition of parts holds good also 

 for the higher divisions of the Tunicata, amongst which the Cyclo- 

 myaria and Thaliada appear as swimming forms, moving themselves 

 by the action of the walls of the body. Their body, which is on the 

 whole of a cylindrical form, has its incurrent orifice at one pole of the 

 long axis, and the orifice leading from the cloaca at the other or 

 aboral pole. 



§301. 



Complications of the external form of the Tunicata appear in 

 connection with the very common process among them of a sexual 



