MONOGRAPH 



OP 



THE CRAG POLYZOA 



INTRODUCTORY OBSERVATIONS. 



Before proceeding to give an account of the fossil remains of a class of animals of 

 which so little is generally known as of the Polyzoa, it will be necessary, if only for the 

 due understanding of the descriptive terms employed, by those who may be wholly 

 unacquainted with the subject, to say a few words respecting their structure and affinities. 

 No very detailed remarks, however, are required for that purpose, and it is the less 

 necessary, perhaps, to enter fully into the subject, seeing that it has been almost exhausted 

 in the admirable Monograph of Professor Allman, on the " Fresh Water Polyzoa/' to 

 which the reader, who is desirous of further information, cannot do better than refer. 

 But as that author's observations, after he has explained the structure and relations of the 

 Polyzoa as a class, are confined more especially to that division of them which forms the 

 subject of his own memoir, and which difiers in several important particulars from those 

 divisions to which the subjects of the present work belong, some additional details referring 

 more directly to the latter, seem to be called for. 



Thirty-one years ago, Dr. Grant, in some " Observations on the Structure and Nature 

 of Plustrse," drew, for the first time, a distinction between the animals inhabiting those 

 growths, and the Sertularian, or Hydroid Polypes, with which they had previously been 

 associated. Shortly afterwards, the observations of M. Milne Edwards not only showed the 

 propriety of this distinction, but indicated also the still more important fact of the close 

 relationship, in many respects, between the structure of the Ascidian Molluscs and that 

 of the animals in question, which he thence proposed to distinguish by the name of 

 " Holypes tuniciers." 



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