OCELLATE HYDRO-MEDUSA. 37 



The hydranths drop off, and the tips of the branches grow 

 very rapidly, sending off long contorted transparent pro- 

 longations. 



By changing a specimen from a small quantity of water 

 to a larger quantity, the hydranths can be made to grow 

 again, and this process can be repeated several times. 



VII. THE STRUCTURE OF AN OCELLATE HYDRO- 

 MEDUSA. 



(Mnemopsis Bachei.) 



As the small size and flat shape of the medusa stage of 

 the hydroid described in the last section render it some- 

 what unfavorable for laboratory work, the student should, 

 before studying it, make himself acquainted with the 

 structure of one of the larger hydro-medusas. 



The species which is shown, magnified ten diameters, in 

 Fig. 21, is very abundant during the summer along the 

 Atlantic coast, and it is therefore a good form to select for 

 study. Specimens may usually be procured by dipping 

 at the surface of the ocean on quiet evenings with a sur- 

 face-net. The net should be made of medium bolting- 

 cloth, and should be about three inches deep, with a wire 

 rim about six inches in diameter, fastened to a short 

 handle. In using it, fill a bucket about half-full of fresh 

 sea-water, and after dipping or skimming gently along the 

 surface of the ocean for about a minute, insert the net into 

 the bucket, and wash it in the nvater to dislodge any me- 

 dusre which have adhered to it. This process of dipping 

 and washing should be kept up for an hour or more ; and 

 in washing the net, care should lie taken that the rim does 

 not dip below the surface of the water in the bucket, as 

 valuable specimens might thus be dipped out of the bucket 

 and thrown away. 



