EELATIONSHIPS OF THE FIXED AND FREE TUNICATA. 183 



drical organ — the ' violet-line' of M. De Blainville — about half the length of the 

 endostyle. These may possibly be renal, but I am not aware that any definite 

 office has been assigned to them, though corresponding structures certainly occur 

 in numerous other cases also. They consist of a membraneous envelope, contain- 

 ing an abundance of large cellular bodies, with transparent spherical granules of 

 uniform size, both within and amongst the cells. 



" The testis consists of a number of long tubular cseca, forming an elongated 

 whitish mass, lying immediately beneath the brownish-yellow stomach and in- 

 testine, and terminating anteriorly in a fine ejaculatory duct, which opens into 

 the branchial chamber a little behind the vent. The gemmiparous zooids are de- 

 veloped singly." 



Explanation of Plate IX. figs. 1, 2. 



ig. 1. Vertical section of a single system of Botri/llus, to show its relationship to Pyrosoma. 

 a'. Floor of the cloacal chamber. 

 a. Surface of attachment. 

 h'. Lateral extension of the ascidiarium. 



6. Border of the cloacal opening, in the formation of which the zooids themselves take part. 

 Fig. 2. Theoretical diagram, illustrating the morphological difference between Chondrostachys A, 

 as a social, and Pyrosoma B, a compound Pelagic Tunicary. 



a. Summit of the axis of Chondrostachys, corresponding with the apex of the free Pyrosoma. 

 a'. Cloacal side of the apex of Pyrosoma, equivalent to the point of fixity of Chondrostachys. 



b. Growing circumference, or border of the cloacal opening of Pyrosoma, considered as an antro- 

 version of the axis or ascidiarium. 



The arrows indicating the orifices of ingress and egress show that there is no necessary in- 

 version or alteration of the relations of the zooids, either as to each other or to the 

 ascidiarium, in the theory of antroversion, as applied to Pyrosoma, or of retroversion as 

 regards Chondrostachys. 



VOL. XXIII. PART 11. 6 E 



