A CONTRIBUTION TO THE MORPHOLOGY OF 
PENNARIA TIARELLA McCRADY. 
MARTIN SMALLWOOD. 
Tue following paper was undertaken at the suggestion of 
Dr. Charles W. Hargitt, to whose kindly criticism and sugges- 
tions I am under special obligations. I am also under obli- 
gations to Dr. C. O. Whitman for courtesies at the Marine 
Biological Laboratory, where in part the work was done. 
The purpose of the paper is an investigation of the more 
fundamental morphological features of this hydroid, and the 
development of the Medusa and origin of the sex cells. 
In the work of former students of the Pennaridz of the 
United States little attention was directed to other than 
external characters and classification. The early contribution 
of McCrady (10) contains the best general account that has 
been given of this species. L. Agassiz’s (1) description of 
P. gibbosa is equally excellent for that species; yet in neither 
is there any reference to features of structure, which have later 
assumed a degree of importance not then recognized. 
Methods. — For killing and staining a variety of methods 
was tried, but those more generally familiar gave usually the 
best results. Fresh Pennaria were immersed at once in corro- 
sive-acetic, picro-acetic, Kleinenberg, and Perenyi’s fluid. The 
best results were obtained from corrosive-acetic and picro-acetic. 
An excellent differential stain was secured by staining zx foto 
and then extracting with acid-alcohol. Borax carmine was found 
to be the best stain with material fixed in picro-acetic. There 
seemed to be no choice of clearing agents, either xylol or 
turpentine was satisfactory. Excellent whole mounts of the 
hydroid were made by staining for a considerable time — two 
days —in borax carmine, then extracting with acid-alcohol. 
The specimens were next cleared in clove or cedar oil for 
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