INTRODUCTION. Xvil. 



The walls of the alimentary canal are glandular, and a 

 separate digestive gland may exist in the form of a system 

 of minute tubes ramifying over the intestine, and opening 

 into the stomach. A renal organ is present occupying one 

 loop of the intestine, another loop encloses the reproductive 

 organs (see PL A, fig. 4, and PI. B, fig. 10). 



The Ascidian is hermaphrodite through not usually self- 

 fertilising. The ovary and testis are both of them ramified 

 tubes, and their cavities represent a part of the original 

 coelom of the embryo which is not developed into any 

 continuous body cavity in the adult. The reproductive ducts 

 are continuous with the walls of their organs, and lead along- 

 side the intestine to terminate in the atrial cavity close to 

 the anus. In some Ascidians reproductive organs (gonads) 

 are present on both right and left hand sides of the body, 

 and in yet others there are several or even many (in 

 Polycarpa) complete sets of male and female gonads attached 

 to the inner surface of the mantle on both sides of the body, 

 and projecting into the peribranchial cavity. The condition 

 of the reproductive system affords valuable characters in 

 classification arid the diagnosis of genera and species. 



This brief sketch* of the structure of a typical Ascidian 

 will probably serve as an explanatory guide to the technical 

 descriptions of species and diagnoses of genera and other 

 groups that occur in the body of this catalogue. 



In Plate B, I have added a few figures illustrating the 

 combination of " Ascidiozooids ' to form colonies in the 

 Compound Ascidians. Thus figure 5 shows two Ascidio- 

 zooids with a common cloacal aperture ; figures 6 to 8 show 



* A somewhat fuller account on the same lines will be found in the Article " Tunicata" 

 of the Encyclopedia Britannica, 9th Edn. ; and a much more detailed statement is given 

 in the Introduction to the " Challenger" Report on the Tunicata. After that reference 

 must be made to the original memoirs of Milne-Edwards, Huxley, Savigny, Lacaze- 

 Duthiers, Giard, Traustedt, Sluiter, Herdman, Van Beneden and Julia, Roule, and many 

 others. 



