HALOCORDYLE COOPERI. 211 



of the tentacles in the hydranths of one and the same colony 

 is not absolutely invariable, although the greater number of 

 the polyps conform to the same plan ; but aberrations do 

 occasionally occur. The variability of the polyps in this respect 

 in the same colony has not been investigated, but only the 

 general average arrangement in the colony has been recorded. 



Thus the average arrangement of the tentacles in colony 

 " 2," Scottsburg, is for four capitate tentacles around the 

 mouth to alternate with four at a lower level ; in colony " 3 " 

 the lower verticil in the majority of the polyps consisted of 

 six or seven tentacles. 



The sex of the colonies could only be determined when 

 gonophores were present. When a distinct oral verticil could 

 be distinguished, it will be noticed that in no case did it 

 normally consist of more or less than four tentacles. The 

 lower or basal whorl is the variable one; typically it is 

 composed of four tentacles alternating with the four of the 

 oral verticil ; but it may consist of five, six, or even seven 

 tentacles. In three cases out of the forty-two no definite 

 verticils could be distinguished, and the average numbers of 

 the tentacles in these three colonies, ''7," "41," "40," were 

 twelve to thirteen, nine to ten, and ten to eleven respectively. 



To show in the form of a diagram the distribution of the 

 synnnetry of the arrangement of the tentacles in the population 

 it will be necessary to arrange a scale of symmetry which, of 

 course, must be empii-ical. 



Let (" ") be the condition of having thirty-two to twenty 

 capitate tentacles irregularly scattered. This has not been 

 observed in H. coo peri, but it occurs in Ponnaria and H. 

 tiarella. 



Let (" 1 ") be the reduced number of thirteen to nine 

 tentacles irregularly scattered. Observed in three colonies 

 (about 7 per cent.). 



(" 2 ") Colonies with the average arrangement of four in 

 an oral verticil, and six to seven tentacles below, which may 

 be arranged in more or less of a verticil. Observed four 

 times (about 9"5 per cent.). 



