588 THE AMERICAN NATURALIST. [Vol. XXXVII. 



break through the perisarc ; a perisarcal sheath begins to be 

 secreted about them, and their cellular structure becomes less 

 distinct. As the hydroid now continues to grow, the perisarc, 

 which in the younger stage was somewhat closely applied to the 

 base, now becomes separated from it by a considerable distance, 

 forming a loose corrugated sac over the proximal end. With 

 the withdrawal of the coenosarc, the papillae also sever their 

 connection with the coenosarc. The living cellular portion now 

 being separated from that of the main stem and hence devoid of 

 any means of nourishment, disintegrates, leaving merely a hollow 

 perisarcal tube in its place. Upon examining sections of the 

 filamentary rhizoids, we find that they are mere hollow tubes of 

 perisarc, terminating in an imperforate and somewhat clavate 

 extremity. There thus seems to be no doubt that these rhizoids 

 are but the products of secretion of the papilliform processes. 



Allman. ('71, p. 209) referring to them says that these proc- 

 esses "have never been seen to act as organs of adhesion nor 

 have we yet any evidence of the office they may serve in the 

 economy of the animal, but it is impossible not to recognize 

 in them structures having a close relation to the filaments of 

 attachment which are given off from the stem a little lower 



(c.) Hydranth. — The most striking histological feature of the 

 hydranth is the presence of numerous and highly developed 

 gland cells (Figs. 6, 7). The development of this remarkably 

 specialized structure leaves little doubt that the function of diges- 

 tion devolves especially upon this part of the hydroid cavity and 

 that there are here structures set apart for the distinct purpose 

 of secreting the digestive fluid. In the distal part of the 



and complicated folds or ridges whose surfaces are covered with 

 these digestive gland cells. In the intervals between the ridges 



takes a denser stain than that of the surrounding cells. ? The 

 free ends of the gland cells are seen projecting out into the 

 hydranth cavity where their walls may often be seen raptured 



(Fig. 6, a.) Often the gland cells may be seen entirely separated 



