INTRODUCTION. 



AS this is neither a treatise on the anatomy of serpent-stars, nor a 

 critical essay on their classification, no parts are treated of that 

 do not hear directly on specific and generic relations. 



Without entering npon the metaph^'sical questions of the existence, 

 limits, and relations of genera and of species, I have tried, as far as 

 might be, to keep the characters strictly separate, and not to mingle 

 generic marks in specific descriptions, or vice versa. 



Although the names which are commonly used for the different 

 organs by no means express their true liomologies, but, on the con- 

 trary, are mostly names applied originally to anatomical parts among 

 Vertebrata, I have nevertheless held to the received nomenclature, 

 because any change would have to be explained by a chapter on the 

 homologies of Radiata, which could not be here admitted. 



Jlethod of describing. — An Ophiuran, to be "in position," should be 

 placed with the mouth down ; then, horizontally towards the points 

 of the arms, is outwards ; horizontally towards the mouth, is inwards ; 

 vertically towards the back, or upper surface of the animal, is vjnrards; 

 vertically towards the mouth and under surface of the animal, is down- 

 wards. It is essential that these directions should be borne in mind. 



It should not be foro-otten that these animals are, at different stao;es 

 of growth, quite unlike in the number, size, and proportions of their 

 parts ; and that even the full-grown ones present a compound of 3'oung 

 and of adult characters, since the outer part of the arm is made up of 

 joints not yet fully developed. Hence it is necessary, in determining 

 different species, that specimens of the same size should be compared 

 with each other. Considering these differences, it is proper to state, 

 that, when '• arm-plates," " arm-joints," &c. are spoken of, without fur- 

 ther qualification, in the following descriptions, it means only such as 

 are near the base of the arm, and are therefore fully grown. Also, 

 where the " length " or the " breadth " of a part is mentioned, the 

 greatest length, or breadth, is referred to. 



The millemeter is always used as the unit of measurement ; there- 

 1 



