30 RELICS OF PRIMEVAL LIFE 



the types best known in the modern seas ; while 

 the Brachiopods, and probably some still simpler 

 forms, are in great comparative excess. The indi- 

 vidual specimens are also of small size, as if these 

 creatures were but insinuating themselves on the 

 arena of life in insignificant and humble forms. So 

 far as yet known, the lowest groups supposed to 

 be allied to the Mollusks, the Ascidians or Sea- 

 squirts, and the Sea-mosses (Polyzoa), do not ap- 

 pear ; but they may have been represented by 

 species which possessed no hard parts capable of 

 preservation. 



This leads us to the consideration that while all 

 the Crustacea necessarily possess some kind of crust 

 or external skeleton, the Mollusks are very differ- 

 ent in this respect. While some of them have 

 ponderous shells, others even of the highest forms 

 are quite destitute of such protective parts. This 

 again leads to a curious question respecting the 

 armature of the Trilobites. Some of these, even 

 of the larger species, have strong and formid- 

 able spines, like those of the King-crabs and 

 other modern Crustaceans. Now in the modern 

 species we know these organs to be intended to 

 defend their possessors against the attacks of fishes 



