ZOOGENESIS 



of following different developmental lines from (approximately) 

 the gastrula onward. The ancestral line of each radiates from the 

 gastrula, and the groups were therefore never connected by inter- 

 mediate types of adult animals, nor was any one derived through 

 any of the others. The appearance of all was probably simul- 

 taneous or nearly so, and also simultaneous with that of the types 

 not shown in this figure (see figs. A-C). 



4. Mainly bilateral animals which, although remaining prac- 

 tically independent of each other, form colonies through asexual 

 reproduction by transverse fission. 



If we place these four structural types, represented by (i) 

 jointed tapeworms, (2.) flukes, (3) nematodes and most turbel- 

 larians, and (4) Microstomum^ at equal intervals on the circum- 

 ference of a circle (fig. E, p. ? 54) we find that there are four other 

 groups of animal forms which structurally may be interpreted as 

 intermediate between them, or rather which combine in a single 

 type the characteristic features of the forms on either side. The 

 animal types included in this second series of four have no trace 

 whatever of radial symmetry, so they may be arranged on a 

 circle within the circle on which the Vermiformes are placed. 

 These types present the following characters: 



I. Animals which are sharply segmented or jointed, like the 

 jointed tapeworms, and also in their growth develop internal 

 buds more or less after the fashion of the flukes, leading to the 

 development of a coelom. The animals which answer to these 

 specifications are the annelids or jointed worms. 



2.. Animals which are unsegmented, like the nematodes and 

 turbellarians, but possess an internally budded coelom, thus in 

 this respect corresponding to the flukes. These animals are the 

 sipunculids. 



3. Animals which are solitary and unsegmented without a 

 coelom, like the turbellarians and nematodes, but show abundant 

 asexual reproduction like Microstomum. These animals arc the 

 rotifers. Possibly the priapulids belong here. 



4. Animals which are segmented but without a coelom, like 

 the jointed tapeworms, but less completely unified and not sub- 

 ject to a continual loss of the units, as in Microstomum. Probably 

 the graptolites belong here. 



[2-55] 



