ANIMAL EVOLUTION 



537 



A second readjustment of the same 

 nature as the first would be inevitable. 

 This would result in the appearance of 

 four different types, each of which would 

 combine the characters of two of the 

 preceding types, and of three of the types 

 first mentioned. 



Three such intermediates seem to be 

 clearly indicated, in (i) the polyzoans (or 

 bryozoans), colonial and not at all, or 

 very imperfectly, ccelomate, between the 

 rotifers and the graptolites; (z) the 

 arthropods (insects, crustaceans, spiders, 

 etc.) with a segmented body like that 

 of the annelids, but divided into two or 

 three units showing division of labor (in 

 the insects one, the head, controlling and 

 directing, one, the thorax, locomotor, 

 and the third, the abdomen, performing 

 the digestive, reproductive, and other vital 

 functions) as in an animal colony, with 

 a poorly developed ccelome, with abun- 

 dant traces of asexual reproduction (poly- 

 embryony, parthenogenesis, fragmentation 

 of larva;, etc.), with a tendency to form 

 colonies showing a division of labor 

 among the (dissociated) individuals (as 

 in the Hymenoptera and the termites), 

 and sometimes even forming plant-like 

 colonies (Thompsonia); and (3) the mol- 

 lusks, always solitary, like the priapulids 

 and sipunculids, with a highly developed 

 ccelome, and with traces of segmentation 

 (in the larva;) suggesting the annelids. 

 The last (4) group should be solitary, 

 with a ccelome, without well developed 

 segmentation, and with an indication of 

 colonial structure. It is possible to place 

 the nemerteans here, on the assumption 

 that their remarkable powers of reparation 

 are in reality related to budding and re- 

 creation of individuals of the ccelenterate 

 type. 



There is still a condition of unstable 

 equilibrium, for in each of these four 

 groups one of the original elements is 



lacking. A third readjustment would 

 be necessary to recombine all of the main 

 features characteristic of each of the 

 original four types. 



Four animal groups appear to be the 

 result of such a readjustment. (1) The 

 echinoderms combine a reduced body 

 consisting of five half segments more or 

 less of the arthropod type with a highly 

 perfected ccelome as in the mollusks. 

 (z) The arrow-worms or cha;tognaths 

 suggest a relationship with the mollusks, 

 and also with the nemerteans. (3) The 

 phoronids suggest a relationship with the 

 polyzoans, although the colonial habit is 

 reduced to the budding off of new indi- 

 viduals, and have a well developed ccelome 

 as in the nemerteans. (4) The brachio- 

 pods or lamp-shells suggest both the 

 polyzoans and the shell-bearing arth- 

 ropods. 



While by this third readjustment all 

 the four original features are recombined 

 in each animal type, the balance between 

 them is imperfect, for in each case the 

 influence of one of these features is greatly 

 overshadowed by the influence of the 

 other three. 



A fourth readjustment would correct 

 this imperfect balance and result in the 

 appearance of four animal types all very 

 much alike. 



There are four types which seem to 

 belong here, the tunicates, the cepha- 

 lochordates, the balanoglossids, and the 

 pterobranchiates. The tunicates seem to 

 be in line with the polyzoans, while they 

 also suggest both the brachiopods and the 

 phoronids. The cephalochordates clearly 

 stand in the cestode-arthropod line, and 

 at the same time show unmistakable 

 affinities with the echinoderms. The 

 balanoglossids, with no trace of asexual 

 reproduction, may be considered as in line 

 with the flukes and mollusks, and between 

 the cha;tognaths and the echinoderms. 



