6 ANTARCTIC MARINE 



Incidentally there is raised the very interesting question whether 

 Terschellingia polaris may not be the free-living form of a dimorphic 

 species having a parasitic stage in some higher antarctic form. It is 

 the writer's impression, based on a very considerable amount of obser- 

 vation, that numerous free-living nematode forms, marine as well as 

 land and fresh water, belong to such dimorphic species. 



The following diagram illustrates the nature of the formula used in 

 the tabulation of the various necessary measurements : 



Fig. 2. Diagram of the descriptive decimal formula used for nematodes ; 6, 7, 

 8, 10, 6 are the transverse measurements, while 7, 14, 28, 50, 88 are the correspond- 

 ing longitudinal measurements. The formula in this case is : 

 7. 14. 28. 50. 88. 

 T! 8. 10. 6. 



The unit of measurement is the hundredth part of the length of the 

 body, whatever that may be. The measurements become, therefore, 

 percentages of the length. The absolute length is given in millimeters 

 as a final non-paired term. 



The measurements are taken with the animal viewed in profile; the 

 first are taken at the base of the pharynx, the second at the nerve-ring, 

 the third at the cardiac constriction or end of the neck, the fourth at 

 the vulva in females and at the middle (3/) in males, the fifth at the 

 anus. The formulae represent the average of several specimens. 



When the specimens were received from Mr. James Murray, the 

 biologist of the Shackleton Expedition, they were in formalin. They 

 were next treated with cold concentrated solution of mercuric chloride, 

 then stained with Mayer's acid carmine and finally examined in balsam. 

 It is W 7 ell to bear these facts in mind in reading the measurements, as 

 both the relative and absolute measurements vary somewhat with 

 various methods of fixation and preservation. The number of speci- 

 mens and their state of preservation is noted at the end of each de- 

 scription, and from the data a rough guess may often be made as to 

 the abundance of the species and the relative frequency of the sexes. 



By the use of suggestive conventional signs the formulae are made 

 to convey considerable additional information. Thus the formula on p. 

 7 indicates that the cuticle is traversed by rather coarse transverse striae, 5 



* Formula line of short dashes. See table, p. 7. 



