SUSCEP'I Hill. I I Y l\ IIYDKOIDS. I 03 



hydranth of Pennaria tiarelln will serve as an example <it" the 

 course of disintegration in the hydranth of a tuhularian hydroid. 

 In full grown hydranths in KNC w/8oo to 777/400 approximately, 

 in neutral red, or in KNC after staining with neutral red to make 

 the death changes more clearly visible, disintegration begins 

 in the ectoderm at the tips of the proximal tentacles and pro- 

 gresses basipetally, usually at about the same rate in each tentacle 

 (Fig. i). Some time later, usually when disintegration has 

 progressed over one to tw r o thirds the length of the proximal 

 tentacles, it begins in the ectoderm at the tips of the distal ten- 

 tacles and progresses basipetally in these also, reaching the 

 bases of both series of tentacles at about the same time. At, 

 or often somewhat before this time, ectodermal disintegration 

 begins at the tip of the manubrium and progresses basipetally 

 over the manubrium and body of the hydranth (Figs. 2 and 3). 

 The short stalk below the hydranth differs in relative suscepti- 

 bility in different cases, and this difference, as in the case of the 

 'stalk' of Hydra (Child and Hyman, '19) apparently depends on 

 the occurrence or non-occurrence of contraction. In cyanide, 

 particularly the higher concentrations, this stalk usually con- 

 tracts and in such cases its ectoderm disintegrates at about the 

 same time as that of the mouth region (Fig. 2), but where con- 

 traction does not occur, it disintegrates only after ectodermal 

 disintegration of the body is completed (Fig. 3). 



In general the entoderm disintegrates later than the ectoderm, 

 except near the mouth region. Since the agent must pass through 

 the ectoderm to reach the entoderm, this difference is perhaps 

 not significant, though it may be noted that in neutral red the 

 entoderm stains almost as rapidly as the ectoderm and may 

 stain much more deeply than the latter before death occurs. 

 In the proximal tentacles the solid column of entodermal cells 

 shows a basipetal susceptibility gradient, like the ectoderm, 

 disintegration beginning at the tip of the entodermal column 

 when ectodermal disintegration has progressed half. way more or 

 less to the base (Fig. i). In the distal tentacles the difference in 

 susceptibility between ectoderm and entoderm is similar. In 

 the mouth region the entoderm may disintegrate as early as, 

 or sometimes earlier than the ectoderm, perhaps because the 



