THE PLUTEUS OF LAGANUM SP. 



BY GRACE MEDES. 



The Laganum plutei used for this study were obtained by Dr. D. H. 

 Tennent, November 1913, at Badu Island, while with the expedition 

 sent out to Torres Straits by the Department of Marine Biology of the 

 Carnegie Institution of Washington. I am indebted to him, not only 

 for the material, but for constant assistance throughout the work. 



The plutei preserved by him are of two ages, 29 and 55 hours respec- 

 tively. The former were fixed in Flemming's stronger chromo-aceto- 

 osmic mixture for 6 hours and preserved in alcohol; the latter, which 

 were designed solely for study of the skeletal structures, were killed 

 in fresh water and preserved at once in alkaline alcohol. 



The 29-hour specimens were used entirely for study of the tissues. 

 They were embedded in paraffin, sectioned, and the sections subse- 

 quently stained by Heidenhain's iron-hematoxylin method. The 55- 

 hour specimens, which were too opaque for study in the whole mount, 

 were gradually transferred to water, left 4 hours in a 5 per cent aqueous 

 solution of potassium hydroxide, and after washing an hour, were 

 dehydrated and mounted in balsam. This method, which proved 

 highly satisfactory for preparing these skeletons for microscopic study, 

 was adapted from a process described by Mr. L. M. Peace (5), who 

 recommends its employment for clearing opaque plant tissues. The 

 chloral hydrate solution suggested in the same article could not be used, 

 as it is so strong a corrosive that specimens treated by it became too 

 fragile for further manipulation. The potassium hydroxide removes 

 just enough of the softer tissues to render the specimen transparent, 

 at the same time leaving it sufficiently hard for subsequent washing and 

 dehydration. The latter process was always done on the slide, to which 

 the specimen was made to adhere by allowing it to dry slightly while 

 in the lower grades of alcohol. 



For detailed study of the more minute structures, such as the entero- 

 coeles, it was found necessary to use reconstructions, which were 

 accordingly made from both longitudinal and transverse sections. In 

 addition to those of the whole animal, separate ones of the lumen of the 

 hydroccele and of its lobes were also modeled. For making the recon- 

 structions Bora's method was employed, with plates of beeswax 

 brought to the proper consistency by mixing with Venice turpentine. 



All of the drawings were made from camera-lucida outlines taken 

 with Zeiss objective | and oculars 4 and 6. Figures 4 to 15 were again 



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