174 MR J. D. MACDONALD ON THE REPRESENTATIVE RELATIONSHIPS 



social Ascidians, as in the Polyzoa, can scarcely be denied ; and upon it is im- 

 pressed a limited law of growth or extension which is subservient to the forces 

 determining the development of the zooids. Thus, while both progress in 

 harmony, there are obvious indications of the co-existence of independent 

 powers. 



The primary fixed point, or, as it were, the potential germ of the ascidiarium, 

 taken in the abstract, will be found to be very differently related to the zooids 

 in the compound as compared with the social Ascidians. Thus, the point of 

 attachment of a PeropJiora or Chondrostachys^ for example, may be looked upon 

 as homologous with the cloacal side of the apex, so called, in Pyrosoma, Plate 

 IX. fig. 1 a\ or to the corresponding part in BotryUus, Plate IX. fig. 2 a', the 

 attached surface of the latter genus, Plate IX. fig. 2 a, being equivalent to the 

 exterior side of the apex in the free Pyrosoma, and to the summit of the axis, 

 produced in Chondrostachys, Plate IX. fig. 1 a, and depressed in Diazona^ which 

 latter genus appears to me to be more conformable with the social than with the 

 compound group. The haemal surface of the zooids, we therefore find, is turned 

 in opposite directions in Chondrostochys and Botryllus, — viz., in the former case, 

 towards the summit of the growing axis, and, in the latter, towards the margin 

 of the encrusting common test, or, in other words, the zooids face outwards in 

 one instance and inwards in the other. Though all the gemmcc augmenting the 

 community are dorsal in both examples given, yet, in BotryUus, and especially 

 in Pyrosoma, they are being continually thrown forward, so as ultimately to 

 be in advance of the parent zooids, whereas in the social group, the anterior 

 aspect of the primary zooid being turned towards the surface, upon which 

 the rudimentary ascidiarium is fixed, the new gemmae arise within extensions of 

 the connecting substance in a truly retrograde direction, — i. e., centripetally, or 

 towards the summit of the central axis thus formed. 



In the beautiful species of Botrylloides, of which there are many in the 

 Australian seas, I have frequently observed a rear-rank of gemmae advancing to 

 usurp the place of their parents, which had for some time previously played their 

 part at the fore ; and indeed it is only in this way that the linear and reticulate 

 cloacae (along the sides of which two rows of zooids are at most to be found), 

 can be produced. 



The bell-shaped antroversion of the "ascidiarium" in Pyrosoma appears to be 

 necessitated by the position of the " cyathozooid," so accurately described by 

 Professor Huxley, and of the cloacal apertures of the four " ascidiozooids" sur- 

 rounding it, as also by the peculiar mode of advance of the gemmae, and the 

 forward extension of these palliovascular stolons, which are in all Tunicata inti- 

 mately connected with the growth and nutrition of the test. 



The proximal surface of BotryUus, being fixed, presents so great antroversion 

 of the mass as happens in Pyrosoma, which, in consequence of being quite free, 



