136 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, NO. 7 
culids, with a highly developed coelome, and with traces of segmenta- 
tion suggesting the annelids. The fourth group should be solitary 
with an indication of colonial structure and a coelome, but without 
segmentation. It is possible to place the nemerteans here by assum- 
ing their imperfect segmentation to be of the Microstomum and not 
of the tape-worm type. 
Turbellarians 
Nomatodes 
A a ar 
AAG Pterobranchiates ~ | hee 
Microstomum Polyzoans Tunicates Vertebrates Balanoglosei ds Mollusce 
Cephalochordates 
Brachiopods Echinoderms 
(Graptolites) Annelids 
Te 
Cestodes 
Fig. 3. Showing the development of the various animal types above the coelenterates. 
There is still a condition of unstable equilibrium, for in each of these 
four groups one of the original elements is lacking. <A third readjust- 
ment (fig. 2) would be necessary to recombine all the main features 
characteristic of the original four types. 
Four animal groups (fig. 3) appear to be the result of such a read- 
justment. The echinoderms combine a reduced body consisting of — 
five half segments of the arthropod type with a highly perfected coe- 
—— 
