DIFFERENTIATION IN HYDROID COLONIES. 



143 



Table IV. 



Measurements in mm. X 95- 



It will be seen that, as the tip of the colony is approached, not 

 only do the hydrocladia possess fewer and fewer hydrothecae, but 

 the dimension of the latter through the mesial nematophore 

 reaches its minimum more and more rapidly. Since the hydro- 

 thecae, once formed, do not enlarge with age, it is clear that for 

 such colonies as this, there is a limit of growth and a specific 

 form.^ 



A. pluma. — This species not only supports the statements 

 made for A. octocarpa, though somewhat less conspicuously, but 

 presents another character (of no value in A. octocarpa) by which 

 differentiation in the hydrocladia may be determined, namely, the 

 length of the hydrotheca through the mesial tooth. Fig. 2 

 presents graphically, in concentrated form, the characters of a 

 single colony so far as they concern the discussion. The measure- 

 ments taken from the drawings are represented in the accompany- 

 ing tables. It should be said, in this connection, that these 

 measurements are subject to a certain amount of correction, 

 owing to the great difficulty experienced at times in obtaining 

 true profiles of the hydrothecae. In hydrocladium no. 25, for 

 instance, the hydrothecae have suffered a degree of rotation 

 which could not be rectified by the means at my disposal. The 

 application of pressure sometimes suffices, but is very liable to 

 produce more serious distortions. Fortunately, the errors which 



^According to Table IV., the length of the hydrothecse does not appear to be a 

 significant dimension in A. octocarpa. 



