HYDROIDS FROM WOOd's HOLL, MASS. 345 



General Anatomy. 



The hydranth is terminated at its free end by the usually 

 conical hypostorae, containing considerable pigment at its 

 highest point (fig. 1, h.). It is pierced by a small mouth-open- 

 ing leading directly into the coelenteron. Immediately below 

 the hypostorae is the set of oral tentacles, ten in number, and 

 placed at regular intervals like radii (fig. 1, o.t.). Ten being 

 the largest number commonly present, I take it to be the 

 normal. These tentacles are one third to one half shorter 

 than those of the lower circle, but are otherwise of the same 

 shape and structure. Tliere are no scattered tentacles on the 

 hydranth, and the lower or aboral set occurs nearly two thirds 

 the length of the whole hydranth from the oral one. They 

 are in no way different from those of the oral circle except 

 that they are longer. There are usually fourteen, though 

 frequently a smaller number has been observed (fig. 1, a. t.). 



In general the tentacles are stouter in appearance than is 

 usually the case in such small polyps. They are slightly 

 enlarged at the end, though there is no knob present except 

 in the young animal, where they are somewhat knobbed. The 

 larger appearance of the tentacles is no doubt due to the 

 prominent ridges of nettling organs which run in circles and 

 short spirals, pushing their cnidocils considerably above the 

 surface. The ectoderm of the tentacles is very transparent, 

 and not easily separated from the mesogloea. The endoderm 

 forms a solid axis through the centre of the tentacle, and in 

 polyps somewhat reduced by fasting, much black pigment 

 collects in these cells, giving the tentacles the appearance of 

 being hollow ; even in ordinary specimens some pigment may 

 be present. 



The gonophores (fig. 1, g.) spring from the hydranth just 

 above the aboral circle of tentacles, and number in adult 

 polyps from one to three, never more than three having been 

 observed. They present some peculiar features, which will be 

 more fully described under reproduction. 



The predominant colour of the hydranth is located in its 



