HYDROID. 



PENNARIA TIARELLA. 



PENNARIA is a colonial, marine animal growing on sea- 

 weed, on the piles of docks, and in similar places. It is 

 a relative of the fresh water polyp, which has already been 

 studied. 



1. The Colony. Examine a portion of the colony with a 

 lens, and note that it consists of a stem with branches, each 

 terminated by a flask-shaped body. This body is called 

 the hydranth or zob'id, and represents a single individual of 

 the colony comparable to an entire hydra. Are all hydranths 

 alike in form and size? What is the general form and char- 

 acter of the colony? Is there any definite order to the 

 branching? How is the colony attached? This basal 

 portion, the hydrorhiza, is really a creeping portion of the 

 stem. Do young stems arise from it? In the stem the 

 central axis, which is the fleshy part of the animal, is called 

 the cosnosarc, and it is surrounded by a horny, dark colored, 

 protective sheath, the perisarc. Does the perisarc cover 

 all parts of the colony? 



Make a drawing of the colony about twice natural size, 

 showing its habit of growth. 



2. Hydranth. Place a portion of a colony in a watch 

 glass or on a slide and with the low power of the compound 

 microscope make out the form and shape of a single hydranth. 

 Are tentacles present? How many? How arranged? Are 

 the tentacles alike? If more than one kind is found note 



