RELATIONSHIPS OF THE FIXED AND FREE TUNICATA. 175 



admits of the full play of this tendency, aided by a more rapid development of 

 the zooids. Moreover, it would appear, that the margin of the external opening 

 of the common cloaca is permanent, progressively advancing with the newly 

 formed zooids, thrusting themselves forwards between it and the parents from 

 which they sprung. Whereas, in Botryllus or Botrylloides, where there is but 

 one effective row of zooids bordering the cloacae at any particular time, the 

 margins of the openings must be continually undergoing repair and decay. 



On comparing an expansion of Botryllus with one of Flustra, or any other 

 polyzoon, it is curious to observe that the zooids lie virtually face downwards in 

 the latter, and face upwards in the former ; and this is certainly one amongst 

 many points of difference existing between the Polyzoa and the Compound 

 Tunicata, while it favours the view that the Polyzoa hold the same relationship 

 to the BracMopoda* that the compound hold to the simple Tunicata, or, to ex- 

 tend the question, that a Gorgonia bears to an Actinia. 



A test which is common to a number of individuals, i.e., an Ascidiarium, 

 affords the first bond of union or community occurring in Tunicata. The next 

 is obviously the establishment of a common cloacal system; and, lastly, as it 

 would appear, the most important condition, truly suggesting the designation 

 compound, is the intercommunication of the Pallio-Tascular systems of the zooids, 

 either as connected with the process of gemmation alone, or with the nutrition of 

 the common test. Systems of intercommunicating stolons, having no connection 

 whatever with gemmation, frequently present themselves in the compound genera; 

 thus, in Leptoclinum and others, they are simple, or simply branched without 

 reticulation ; while, in Botryllus, they are compound, reticulate, and open a com- 

 munication between the zooids. 



The cloacal, like the branchial aperture, may open upon the external surface 

 of the common test, or it may open into a definite common cloacal system either 

 directly or by a tributary canal. The character and arrangement of the common 

 cloacse are of great importance in classification, and it is much to be regretted 

 that so little definite information respecting them is to be found in systematic 

 works. Whenever the opportunity presented itself, I have always endeavoured 

 to unravel their curious schemes of arrangement, and often found it a matter of 

 great difficulty ; but so far as I have been able to generalize them, they are given 

 in the following classification, which will be also found to afford a simple exposi- 

 tion of the leading features of the Tunicata as a whole. 



As a matter of course, many characters in the minor distinctions here em- 

 ployed must, and have been, previously adopted by others. Thus, Mr W. S. 

 M'Leay and Dr Fleming have passed in review nearly all the available characters 

 in the Simple Tunicata, and M. Milne-Ed wards has done the same with the 



* The ventral valve being the valve of attachment in the Brachiopoda as in the Polyzoa. 

 VOL. XXIII. PART II. 3 C 



