THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL. 



Vol. XIV. MARCH, 1893. No. 3. 



Biological Descriptions of Certain Common Hydroid 

 Animals. 



By H. L. OSBORN, 



ST. PAUL, MINN. 



[with frontispikce.] 



In the chapter on the Protozoa (A. M. M. Journal^ 1893) it 

 was shown that a great variety of animal form is possible by vary- 

 ing the form of a single cell and its different parts, but none of the 

 Protozoa attain an}- considerable size. Now, in the competition 

 of animal life size is an advantage to its possessor, other things 

 being equal. Any very considerable increase of size is impossible 

 in a single cell, because, as pointed out by Herbert Spencer, the 

 solid capacity outruns the surface area, and thus, in increasing 

 sizes, the points where food can be taken in soon become too few 

 for the bulk. Hence, to secure the advantages of size, cells are 

 forced to club together and form multicellular bodies. We find 

 in the coelenterate animals the simplest type of multicellular bodies, 

 many of which are easily accessible to large numbers of students, 

 and this present article is written as a key to the cellular structure 

 of some of the representative members of the group. As many 

 of the readers of this yo«r?^a/ live near the ocean or visit it during 

 the summer, I trust they will find it possible to verify upon speci- 

 mens they can find some or all of the points I shall speak of. 



Hints for Practical Study of Hydrozoa. 



A great deal can be done in the study of these animals at very 

 little cost for appliances, but, of course, the best work requires 

 skill and patience and a good compound microscope. 



Firsts in collecting specnnens of marine Hydrozoa from a boat 

 or w^hen in bathing, examine submerged sea-weed or scan care- 

 fully the submerged portions of objects such as piles of wharves, 

 floats, buoys, drift-wood, and remove with knife parts which ap- 

 pear to promise well, placing the material thus obtained in a tum- 

 bler with plenty of sea-water. Give it time to recover from the 

 stimulation you have caused, and if there be any living hydroids 

 in your capture they will presently reward you l3y expanding ; or 



