LIFE IN SECONDARY TIMES 249 



branchs increased that buried themselves in sand, mud, or 

 even limestone, and that remained in communication with the 

 exterior only by means of two long tubes, the siphons, situated 

 in the posterior part of the body, of which one distributed the 

 water on to the branchiae, which then passed it on to the 

 mouth, while the other ejected the water from which the 

 oxygen and food particles had been assimilated, and which 

 carried with it the excreta. 



This change of habit, which, indeed, furnishes us with food 

 for reflection, is not limited to the Lamellibranchs, for it also 

 occurs in the Sea-urchins. The Sea-urchins of Primary times, 

 without any predilection in choosing their direction, crawled 

 among the Algae or on rocks. The shell of the more recent 

 species was divided into ten areas and bore numerous spines. 

 These characteristics have been preserved, but there were 

 others as well — burrowing species, covered all over with fine 

 spikes, which dug their way in the sand in a definite direction. 

 In these species, continually pressed against the covering earth, 

 the shell is flattened around the mouth and forms a true 

 ventral surface ; the ambulacra on this side have taken on an 

 entirely different appearance from those of the dorsal side, 

 and henceforth their tube-feet alone play a part in locomotion. 

 As the excreta are now no longer easily evacuated through the 

 top of the shell, buried as it is in the earth, the anus changes 

 its position from the top to the neighbourhood of the ventral 

 face, thus characterizing a posterior region of the body opposed 

 to that which the sea-urchin carries in front when it is 

 burrowing. Thus a very definite bilateral symmetry (pp. 127 

 and 149) is superposed on the primitive radial symmetry. The 

 mouth at first remained situated in the middle of the creature's 

 ventral aspect and conserved the jaws of the primitive Sea- 

 urchin, though somewhat diminished in size — an example of 

 which is provided by the Clypeastridae. Later the mouth 

 moved almost to the anterior edge of the ventral surface ; 

 the posterior lip advanced, spoon-fashion, in such wise as to be 

 capable of being dug into the muddy sand, and of shovelling 

 it into the sea-urchin's mouth. The useless jaws disappeared. 

 An example is seen in the Spatangidae. During the Secondary 

 era we find all the stages of transition between the ordinary 

 Sea-urchin and this type, and they are so numerous in 

 certain geological layers that they have served to characterize 



