INVERTEBRATE ZOOLOGY. 71 



progress? Study the oldest of these buds. Can you find traces of medusa 

 structure? [of. 35]. 



34. Express bj- drawing the results obtained. As such may be sug- 

 gested : 1) a sketch of an entire branch, only a few times enlarged, drawn 

 from dissecting microscope. 2) An enlarged figure of each sort of polj^p. 

 Make this a perfect and symmetrical diagram, taking the details from several 

 if necessary. Thus one specimen may have the best manubrium, another may 

 show good tentacles, another a perfect hydrotheca, etc. 3) Drawing of de- 

 tails as seen with a high power. Thiis a portion of an enlarged tentacle, 

 showing nettle cells, or a bit of body wall seen in •• optical section " i. e. 

 focused to show the thickness of a lateral wall : the specimen is thus ciit bj- 

 the focal plane of the microscojie. Such detailed drawings as these may 

 be drawn beside (3) and connected by dotted lines with the corresponding 

 part. 



35. Sti((ly of a free nieduHa. These are caught in the open ocean ^^•ith 

 a tow net and may often be found in preserved specimens of tow. In this 

 species thej' are of the .size of pin-heads, disc-slia])ed and bordered by a fringe 

 of tentacles. Search for them as follows: Place a watch crystal of 70 ^f on 

 stage of dissecting microscope. Pipette into this a drop of thick tow. This 

 dilution will serve to isolate the forms, which may then be sorted over with a 

 dissecting needle, looking continually through the lens. In case of fresh liv- 

 ing tow, use sea- water instead of 70^ for the dilution. When one is found, it 

 may be stained and moiinted as in 33. but handled with a piijette instead 

 of a needle. In this, reduce to a minimum the amount of liquid transferred 

 with the specimen. In case a specimen becomes lost, place the crystal ujjon 

 the stage of the dissecting microscope and search as at first. In a well 

 mounted specimen there may be seen : — (1) the manubrium. (2) four radiating 

 canals. (3) four genital masses lying upon the cauala, (4) the tentacles with 

 otocysts at the base of some of them. 



Other Studies of Hydrozoan Forms. 



36. Pennaria is a common form and shows manj' important differences 

 from the foregoing. Prei)are and study in the same way and note. 1) the 

 perisarc does not grow ny over the polyps. l)ut stops short at their bases, leav- 

 ing them unprotected. 2) The medusa-buds do not grow upon a blastostyle. 

 but at the bases of the ordinary polyps, which are of but one kind, all hy- 

 dranths. 3) These buds do not become free, biit remain as reduced (jono- 

 phores, producing the genital products in the place of origin. The gonophores 



