98 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Skr. 



Caudal Zone. — (figs, i, 21, 22,) The location of this 

 zone has already been described. It has been generally 

 known as the "zone of growth," almost ever since it was 

 so called by Schmarda. The zone of growth was first 

 referred to as such by Fritz Miiller, and since his time 

 every investigator who has had an opportunity to study 

 this interesting genus has speculated upon the significance 

 and purport of this unusual structure. Fritz Miiller and 

 Beddard are of the opinion that the zone of growth is a 

 place where new segments originate and where they may 

 be most readily renewed. Beddard assumes that this 

 swollen zone will easily break, and that at this point rapid 

 segmentation is possible. 



Horst (17) does not share Beddard's opinion. He points 

 out that the zone is characterized by a large number of 

 capillaries and figures these vessels (Tab. IV, fig. 40), 

 but suggests no special use for them in this region. Upon 

 what feature of the zone Beddard founds his theory of 

 regeneration is not quite evident. Out of many hundreds 

 of specimens of Pontoscolex collected by the writer, only 

 one was found in which the caudal zone forms the terminus 

 of the body, and here it is evident that the tail-end has been 

 lost. I do not find, however, that Pontoscolexis more easily 

 torn than other worms, and cannot see how the above the- 

 ory explains the formation of this extraordinary structure, 

 as far as known, only once paralleled elsewhere. I have 

 sectioned a number of these zones of growth but have yet 

 to find a single cell in mitosis, though the specimens were 

 young and undoubtedly growing. This certainly seems to 

 indicate that if the caudal zone is a place for active growth, 

 the activity is apparent only under certain conditions, as 

 when the tail is being regenerated, as suggested by Bed- 

 dard. But if regeneration of the tail of Pontoscolex is an 

 occurrence so common that a special organ is required for 

 the work, we should expect to find a large number of 

 worms with broken tails, which is not the case. 



In regard to this zone, I can only say that the specimens 

 examined by Dr. Horst differ materially from those I have 



