250 TOWARDS THE HUMAN FORM 



them. The burrowing Sea-urchins actually swallow the mud, 

 and this mode of nourishing themselves is a factor in their choice 

 of habitat. But, we may ask, is not this in itself a result of their 

 search for security ? 



Among the Lamellibranchs Cardium digs up the soil as if 

 seeking food in it, and may serve as a starting-point which 

 will explain the formation of siphons in organisms such as Solen, 

 the Razor-shell, which only moves upward and downward in a 

 vertical burrow and feeds on floating particles carried thither 

 by the water without any search on the part of the Razor- 

 shell . It is impossible to see anything in its underground habitat 

 beyond the desire for security. This is quite evident in the case 

 of the Pholadidae, which perforate the limestone they cannot 

 possibly eat, and in that of Teredo, which lives in wood. If these 

 animals were thus led to live in seclusion, we may suppose that 

 those among the congeners of their ancestors which did not adopt 

 this way of life were destroyed. This would be a consequence 

 of that struggle for existence, in which only those organisms 

 survived that were able to avoid it, either involuntarily or of 

 set purpose. 



This motley crowd of Invertebrates was dominated by 

 innumerable swimming Molluscs, first among whom we see 

 the Ammonites riding the waves and seated as one might say 

 in their shells, spirally twisted like the horns of Jupiter Ammon, 

 and divided into chambers, whose origin and increasing com- 

 plexity during the entire Jurassic Period we have already 

 described. What purpose can have been served by this com- 

 plexity, which was never produced in the Nautili ? If we 

 admit the assimilation, postulated by Munier-Chalmas, of the 

 straight-partitioned Cephalopoda with the Nautili, and of 

 those having folded partitions with the Spirulae, certain 

 observations become unavoidable. The first of these must have 

 had at least two pairs of branchiae and the second only one, 

 possibly because of a shortening of the body, which was in 

 communication with the outside world only by its anterior 

 extremity, and which thus underwent a kind of cephalization. 1 

 Under these conditions the mantle, increasing its surface 

 by folding, was able to take the place of the second pair 

 of absent branchiae. The folds would become more complicated 

 as the Cephalopod became more active and its potential size 



1 Cf. pp. 219-20. 



