472 LECTURE XX. 



To connect the Tunicata with any of the classes of animals which 

 we have previously considered, it is necessary to revert to the Polypi, 

 for it is in this group of the Radiata that we shall find the animals 

 which have the closest natural alliance with the present class of 

 MoUusca. 



Suppose a Bryozoon to have its ciliated oral tentacula reduced to 

 mere rudiments, and to have the pharynx enormously expanded, with 

 its vascular internal surface richly beset with vibratile cilia ; it would 

 then be converted into an Ascidian, and the transition from the 

 Radiated to the Molluscous series would be effected ; — so easily, as it 

 would seem, and at the expense of so slight a change of any essential 

 structure, that some, according to the importance they may attach to 

 the Zoologist's definitions, groupings, and divisions, might be dis- 

 posed to blame the present entry upon the molluscous type of organ- 

 isation by the class in which the respiratory organ is distinctly 

 developed and the polype-form abandoned. It is chiefly by reason of 

 the external character of the crown of radiating tentacles that the 

 Bryozoa have been placed in the radiated sub-kingdom. Their 

 minute size, fixed position, composite aggregation, and gemmiparous 

 propagation added, doubtless, to the belief in their being Polypi. 

 But relations of size have no value in classification ; the mouse is as 

 good a mammal as the elephant. The choice of their position mainly 

 depends upon the importance assigned to certain phenomena of de- 

 velopment (p. 153). According as the Naturalist calls the Flustra a 

 polype or a moUusk, he exemplifies his mode of interpreting the sig- 

 nification of the difference between the ciliated gemmule of the 

 Flustra and the cercariforra larva of the Polyclinum, of the resem- 

 blance of the bristled coriaceous ovum of the fresh-water Alcyonella 

 to that of the fresh-water Hydra, and of the difference of both from 

 the ovum of any known compound Ascidian. If these significant 

 indications of the fundamental affinity of the Polypes, with the re- 

 tention of the polype-form, and the absence of a respiratory organ in 

 the highest of that class should beget a doubt as to the propriety 

 of calling a Bryozoon a MoUusk, and thereby losing the advantage of 

 the latter term as a definite and intelligible sign of a certain advance 

 of organisation, the comparative anatomist, whilst admitting the full 

 amount of the affinity of the Bryozoa with the Tunicata, and thereby 

 illustrating his view of the Molluscous series as constituting a great 

 parallel branch of the animal kingdom with the articulate series*, 

 may anticipate a verdict in favour of his judgment, in the necessarily 

 artificial mode of successively treating of the different types and 

 grades of organisation, if he should select the compound Ascidians as 



* LXXXIV. p. 269. 



