DIFFERENTIATION IN HYDROID COLONIES. I4I 



statistically thorough. They are rather illustrative of certain 

 facts of differentiation that do exist and are capable of experi- 

 mental analysis. 



A. octocarpa. — A glance at the accompanying camera draw- 

 ings (Fig. i) will acquaint one with the main differentiating 

 characters on which the present study is based. The four series 

 of figures represent hydrothecae from four hydrocladia. That 

 they might be representative of the colony, the latter were 

 selected at various levels on the right of the stem, being nos, 8, 

 25, 41 and 56, in a series of 64,' and care was taken that they 

 should be the longest obtainable and, if possible, not damaged 

 at their distal ends. Similarly, the hydrothecae, drawn in pro- 

 file, were chosen as representative of the whole hydrocladium» 

 being nos. i, 2, 8, usually 12, and the terminal two. 



In the condensed picture of the conditions throughout the 

 colony thus obtained, certain facts stand out prominently : 



1. Each hydrocladium tapers from base to tip.^ 



2. On each hydrocladium, hydrotheca no. i is noticeably 

 smaller than the others, and the mesial nematophore associated 

 with it is especially short. 



3. From base to tip, the openings of the hydrothecae become 

 less oblique to the axis of the hydrocladium. This obliqueness 

 may be shown numerically by measuring the angle made with the 

 axis of the internode by a line drawn tangent to the depressions 

 between the first and second, and the third and fourth, lateral 

 marginal teeth on each hydrotheca (12, Hcl. 8, Fig. i). Two 

 series of measurements from different colonies are shown in 

 Tables I. and II. 



4. The mesial nematophore reaches its greatest prominence 

 toward the middle of the latter and then declines to the tip. This 

 fact is numerically expressed in Table III., which shows the 

 profile width of each hydrotheca to the point of the mesial 

 nematophore on a line perpendicular to the axis of the internode 

 (see Fig. i, hydrocl. 8, hth. 8'). 



^The tip of the colony was wanting. 



^ This, as shown by repeated observations, is independent of the thickening of the 

 perisarc with age. For convenience, neither such thickenings of the walls nor the 

 septal and intrathecal ridges characteristic of the species have been drawn, except in 

 12, Hcl. 2j, where the intrathecal ridge is shown. 



