Tubularian Hydroid. 17 



THE HYDRIFORM STAGE OF A TUBULA- 

 RIAN HYDROID 



(Pennaria tiarella). 



External Anatomy. Examine with a low power, and 

 note how the present specimen differs from the Campanula- 

 rian in its order of branching, in the disposition of its 

 hydrorhiza, and in the termination of the branches of the 

 hydrocaulus. No hydrothecae are present. How do the 

 zooids or hydranths differ from those of the Campanularian? 

 Specialized gonangia are not present, but medusa-buds will 

 be found as globular swellings along the sides of certain 

 hydranths. 1 



Make an enlarged drawing, naming the several parts. 



Examine a single expanded hydranth under a higher 

 power, and locate the manubrium, the terminal mouth, the 

 somewhat irregularly placed " shorter tentacles," and the 

 whorl of u longer tentacles " arranged around the base. Is 

 the number of tentacles constant in different hydranths? 

 Are batteries of nematocysts present? 



Make an enlarged drawing of a hydranth. 



Internal Anatomy. Does the internal anatomy essen- 

 tially differ from that of the Campanularian? Do you find 



1 Hydroids that are thus unprovided with hydrothecae and gonan- 

 gia are united in the group Gymnoblastea, while the Campanularian 

 Hydroids (provided with hydrothecae and gonangia) are united in the 

 group Calyptoblastea. 



