XVI INTEODUCTION. 



It must, however, be clearly borne in mind by the student 

 that, as a general rule, ouly the hard parts, or skeletons and shells, 

 of the animals which are found buried in the soils and rocks 

 composing the earth's crust, are preserved in a fossil state; if, 

 therefore, we desire to understand what kind of animal is 

 represented by the fossil organism in the rocks, we must know 

 at least a little about its living representatives. 



In some instances an attempt has been made, as in the 

 Cephalopoda (Gallery YII), the Brachiopoda, Crustacea, and the 

 Crinoidea (Gallery YIII), to introduce drawings and specimens 

 to illustrate these groups ; and it is intended to carry this on 

 still further in other divisions of the Museum. 



In this Guide to the Invertebrata a brief account is given 

 of the soft parts of each different class, so that the student, who 

 sees in the Geological Galleries the hard parts of these animals 

 placed before him in a fossil state, may learn from the Guide 

 what was the aspect of the living animals of which they once 

 formed a part. 



It may be well to add, that from the delicate and perishable 

 nature of many of the Invertebrata (such as the Nudibranchs 

 and other soft-bodied animals) it is hardly likely that they will 

 ever be found in a fossil state ; the zoological series in the Geo- 

 logical Galleries is not therefore complete. 



We must also mention that the subdivisions in the Invertebrata 

 should not be compared with those in the Vertebrata, for they 

 represent many divisions each more extensive than the whole of 

 the Yertebrata; the individuals are not so large, but they are 

 greatly superior in numbers, whilst their geological record is vastly 

 more ancient. 



H. W. 



