PRIMITIVE ANIMAL FORMS 123 



It is evident that a branched organism would be greatly ham- 

 pered in its attempts to move backwards, all its branches being 

 brushed back in front of its head. Forward movement would 

 result in bending back these branches against the body, and 

 thus prepare for their fusion. There is no reason why these 

 appendages pressed against the body should have been raised 

 from its surface, or spread out laterally, unless they could have 

 been used in locomotion, like the appendages of the Arthropods. 

 In this case they were only necessary because a rigid covering 

 of chitin around the animal's body prevents it from swimming 

 or crawling by means of undulatory movements. The budding 

 therefore is localized at the posterior part of the body, 

 relatively inactive and younger, and formed of non-specialized 

 cells. The new buds are arranged in a straight line behind the 

 old ones, all together forming a body made up in this way of 

 segments placed end to end. The posterior end of the body 

 of the embryo is always completely formed at an early stage 

 by a special segment, constituting a veritable rearguard of 

 sensitive tissue adapted to protect the young animal from 

 contacts to which it may be subjected. 



This last segment or telson is always the second one to form ; 

 the others develop immediately in relation to it, so that the 

 youngest segment of the body is always the one before the 

 last. This short description suffices to give us the basis of the 

 embryogeny of all animals with segmented bodies : Arthropods, 

 Annelid Worms, and even Vertebrates. In the latter the body 

 segments, whose bounds are marked by the vertebrae, are also 

 formed one by one at the back of the body, progressing from 

 a terminal region corresponding to the telson. 



The essential characters of the evolution of segmented 

 animals are thus outlined. At the beginning of their existence 

 they consisted of a single segment, which, from its constitution, 

 has been capable of budding at the posterior extremity, so that 

 the formation of segmented animals has been very precocious and 

 rapid. It is possible that the merids that gave rise to them were 

 originally similar, and that their integument was formed 

 externally of a layer of cells with vibratile cilia ; but this 

 initial type was soon resolved into two others. In one, the 

 superficial cells produced a solid coat, 1 thick enough to glue the 



1 Composed of a special substance of a horny consistency, called chitin, 

 derived from cellulose by the substitution, for one or more atoms of hydrogen, 

 of a nitrogen radical. 



