^^ ZOOGENESIS |S^ 



a highly developed coelom, and with traces of segmentation sug- 

 gesting the annelids. 



4. The fourth group should be composed of solitary animals 

 with an indication of colonial structure and a coelom, but with- 

 out segmentation. It is possible to place the nemerteans here by 

 assuming that their imperfect segmentation is of the Microstomum 

 and not of the tapeworm type. 



Within this third series of animal types there is a fourth series 

 bearing the same relation to the third series that the third does 

 to the second. This fourth series of animal types includes the 

 following: 



I. The echinoderms, combining a reduced body consisting in 

 the adults of five half segments more or less of the arthropod type 

 with a highly developed coelom as in the mollusks. 



2.. The arrow-worms or chxtognaths, suggesting a relation- 

 ship with the mollusks, and also with the nemerteans. 



3. The phoronids, suggesting a relationship with the poly- 

 zoans but with a well developed coelom and with the colonial 

 habit reduced to the budding off of new individuals. 



4. The brachiopods or lamp-shells, suggesting both the poly- 

 zoans and the barnacle-like arthropods. 



In this fourth series of animal types the features characteristic 

 of each of the four groups of types in the first series all occur in 

 varying proportions . A fifth series of four, bearing the same rela- 

 tion to the fourth that the fourth does to the third, would there- 

 fore be composed of types which would be structurally very 

 much alike. 



We appear to find such a series in: 



I. The tunicates, which seem to be in line with the polyzoans, 

 while they also suggest both the brachiopods and the phoronids. 



t. The cephalochordates, which clearly stand in the cestode- 

 arthropod line and at the same time show indubitable affinities 

 with the echinoderms. 



3. The balanoglossids, with no trace of asexual reproduction, 

 which may be considered in line with the flukes and mollusks 

 and between the chastognaths and the echinoderms. 



4. The cephalodiscids, which seem to fall between the chaetog- 

 naths and the phoronids. 



These four very distinctive, but structurally closely related, 

 types all show marked affinities with the vertebrates. It is pos- 

 ts?] 



