— i5 — 



their well being upon a free and uninterrupted interchange 

 of substances within the body and between the body and 

 the exterior, the development of an internal or an external 

 skeleton, previously possible only in animals of extremely 

 small dimensions, become practicable in animals of any size. 



Undoubtedly the most perfect of all the coelomate animals 

 are those in which a marked tendency traceable in the acoelomate 

 types is carried to its logical extreme and all of the body pro- 

 toplasm is incorporated into active bodily tissue with no inert 

 mesodermal or other structures remaining. But these animals 

 (the sipunculids, priapulids, molluscs and annelids), like the 

 cœlenterates, proved victims of their own perfection and led to 

 nothing further. Advance was through those types in which 

 more or less inert tissue was retained which was so modified 

 as to serve for supporting the body or appendages or for the 

 attachment of muscles and the coordination and amplification 

 of muscular action. 



The animals with a more or less developed skeleton or its 

 equivalent fall into two main groups, those without and those 

 with gill clefts. The animals of this type without gill clefts are 

 of two classes, those in which the body is unsegmented 

 externally and internally is divided into not more than three 

 usually incompletely differentiated segments and which have 

 no respiratory organs and no true superficial skeleton (the 

 nemerteans, Phoj^onis^ the brachiopods and the ch^tognaths), 

 and those w^hich have the body abruptly divided externall^^ 

 into five or more segments and possess respiratory organs and 

 an articulated superficial skeleton (the echinoderms and 

 arthropods). 



The animals with gill clefts are divided into tv\'o classes, 

 those in which the nerve cord is ventral with sometimes an 

 additional dorsal (Enteropneusta), and those with a dorsal nerve 

 cord only; these last may be quite without a definite head or 

 limbs (tunicates and Cephalochorda), or there may be a highly 

 developed head and brain combined with highly specialized 

 limbs (Vertebrata). 



Briefly stated, the simpler plants, carrying forward the 

 fundamentally elongate to linear form of the unicellular types, 

 gradually and uninterruptedly developed and expanded into 

 highly complicated multicellular types. 



(400) 



