OBSERVATIONS ON POLYZOA. 203 



to the dorso-ventral diameter of the tube are plain enough 

 in the synopsis, and need no explanatory remarks. This, 

 however, is a fluctuating character, while all the above 

 mentioned are incremental characters. 



The breadth of the muscle was measured near the 

 lophophore ; and the breadth of the tube from the brachial 

 collar to the junction of the arms with the tube. 



The fluctuation in Cristatella is due to the sudden in- 

 crease in the breadth of the tube in this genus, and not to 

 any decrease in the absolute breadth of the muscle, com- 

 pared with what it was in Pectinatella. 



Brachial Retractors. The breadth was measured 

 across the tube, as in the oesophageal retractors. All the 

 characteristics of the retractors, as read in the synopsis, 

 are directly traceable to the increasing evagination of the 

 polypide. 



As the polypide protrudes farther out of the coenoecium 

 and the invaginated fold disappears, the oesophageal and 

 brachial retractors lose the support which it afforded them 

 in Fredericella ; and the increase in the size and diffu- 

 sion of their filaments and bases is to compensate for this 

 loss, and to enable them to govern the motions of the 

 crest with their former facility. 



The increment of the number of the bands of the o-as- 

 trie retractors, however, although due to the same causes 

 that bring al)out the increase in the degree of evagination, 

 are more directly affected by the greater freedom given to 

 the alimentary canal by these changes. In Fredericella, 

 the number of bands is at its minimum, because the stom- 

 ach is confined in a narrow tube ; and in Pectinatella and 

 Cristatella at its maximum, because they are required to 

 confine the alimentary canals of the different polypides, 

 and prevent them from interfering with each other in the 

 common coenoecium. 



ANTERIOR RETENTORS. 



One roiv. These being merely the lowermost row of 

 posterior retentors, and differing from them only in their 

 size, there is necessarily but one row in every species 

 throughout the group. 



