XVI INTRODUCTION'. 



pentagonal particles. The skeletons of the digestive, the 

 aquiferous, and the tegnmentary systems, equally present 

 the quinary arrangement ; and even the cartilaginous 

 framework of the disk of every sucker is regulated by this 

 mystic number. When the parts of Echinoderms deviate 

 from it, it is always either in consequence of the abortion 

 of certain organs, or it is a variation by representation, that 

 is to say, by the assumption of the regnant number of 

 another class. Thus do monstrous Starfishes and Urchins 

 often appear quadrate, and have their parts fourfold, as- 

 suming the reigning number of the Actinodermata, con- 

 sistent with a law in which I put firm trust, that when 

 •parallel groups vary numerically by representation they vary 

 by interchange of their respective numbers. 



In this short introduction I have rather given the gene- 

 ralities of the subject than details of structure, for which 

 I would refer the reader to the excellent account of the 

 anatomy of Echinodermata given by Professor Jones in 

 his Outlines of the Animal Kingdom. I shall conclude 

 by presenting a tabular view of the distribution of our 

 native species. In the first of the two following tables, the 

 numbers of species of each family known to inhabit the 

 several zones of the sea is given ; in the second a view of 

 their distribution in the various provinces of the British 

 seas, with such foreign localities as are recorded. I have 

 divided the marine provinces thus : — I. Thulean, including 

 the Orkney and Shetland Isles. II. Hebridean. III. 

 Scottish eastern coast. IY. English eastern coast. V. 

 English Channel. VI. St. George's Channel. VII. South- 

 ern, the district between Land's End and Cape Clear. 

 VIII. South-west Irish. IX. North-west Irish. X. The 

 Clyde province and North Channel. XI. The Irish Sea. 



