THE CRAG POLYZOA. 



Another independent observer, however, Dr. John V. Thompson, of Cork, was also at 

 work on the same subject, the results of whose researches, apparently commenced in 1820, 

 were not published till December, 1830, in the first part of his " Zoological Researches 

 and Illustrations.'' He, like M. Milne Edwards, recognising the close affinities presented 

 in the structure of the animals to that of the compound Ascidians, was the first to propose for 

 them an appellation wholly independent of their former incongruous allies, the hydroid 

 " Polypes." The term he employed was "Polyzoa," it "being applied" as he says, "to a dis- 

 tinct class of Polypes hitherto in great measure confounded with the Hydroida." But it is 

 to be remarked that he used the word in the singular number, so that the plural term, 

 "Polyzoa," as now employed, though etymologically more correct, is not in reality synony- 

 mous with that of Dr. J. V. Thompson. This fact, which appears to have been strangely 

 overlooked till 1852, may fairly enough be used as an argument in their favour, by 

 those who are inclined to prefer the Ehrenbergian term " Bryozoa." But as this 

 preference, which is still extensively prevalent, more especially on the Continent, is based 

 simply upon the supposed priority of Professor Ehrenberg's appellation, a claim which 

 has been shown 1 to be wholly untenable, it is scarcely likely that British naturalists will 

 refuse the honour justly due to Dr. J. V. Thompson, for what can scarcely perhaps be 

 regarded as a sufficient reason. 



With respect to the general affinities of the Polyzoa as a class, it may be stated that 

 the MOLLUSCAN SUB-KINGDOM admits of subdivision into three great primary groups or 

 provinces, separated by well-marked characters. These are 1. The CEPHALOPHORA, 

 including the Cephalopoda, Gasteropoda and Pidmonata. 2. The ACEPHALA, embracing 

 the LamellibrancJtiata, and, according to many, the Brackiopoda. And 3. The MOLLUS- 

 COIDA, to which belong, according to some, the Brachiopoda, the Tunicata or Ascidioida, 

 and the Polyzoa, whose affinity to the latter, at any rate, is generally admitted ; whilst 

 their relationship to the former is in many respects also unmistakeable. 



Like many of the lower members, more especially of the Tunicate class, the Polyzoa 

 all live in an associated form ; the compound growth or " polyzoarium" (ccencecium, Allm.) 

 being composed of a congeries of distinct individuals, arising by a process of continuous 

 gemmation, from a single primary parent. The colonies thus formed vary infinitely in 

 form, appearance, and consistence. Some are phytoid and erect, whilst others are 

 massive, creeping, or crustaceous. Some, soft and flexible, composed wholly or in part of 

 a horny substance, form delicate growths which yield gracefully to every motion of the 

 waves ; whilst others, firm, rigid, and unyielding as the rocks they live upon, bid defiance 

 to the ravages of time or tempest. 



The class, or rather one order of it, is cosmopolite, its members being found from the 



i On the Priority of the term "Polyzoa" for the Ascidian Polypes. ('Ann. Nat. Hist.,' 2d ser., 

 vol. x, p. 352, 1852.) 



