4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 94 



and in exceptional localities would any copepods be found in this sort 

 of sand. 



In view of these considerations the final answer to the question 

 whether copepods can move about freely in sand is in the affirmative if 

 it is quartz sand, and in the negative if it is shell or coral sand. The fact 

 also that both the shell and the coral sand are calcareous may have 

 some influence upon the copepods. 



As to the mud, its composition is also the most important factor 

 in determining whether it is to be inhabited or not. If it is dense clay 

 there will be no chance of finding copepods within it, and on the 

 other hand if it be sandy the probability of its being inhabited by 

 copepods will become greater as the percentage of sand it contains 

 increases. The upper layers of ordinary mud are more or less floccu- 

 lent, that is they are made up of small flattened flakes or floccules. 

 These are to a certain extent buoyant in water and are so irregular 

 in form that they do not pack together closely but leave sufficient open 

 spaces for the copepods to move about freely. Many of the floccules 

 are also so small and light that they can be easily pushed aside by the 

 copepods during their progress, and so soft that contact with them is 

 not likely to be at all harmful. Consequently, in the mud there is not 

 the same restriction in size that prevails in the sand, and the larger 

 copepods can move about as freely as the smaller ones, possibly with 

 even greater facility. 



Such a life as this, moving about all the time within the confines 

 of the sand and mud, is just as difl:"erent from that of the typical free- 

 swimming forms that frequent the open water as are the lives of the 

 commensal and parasitic copepods. The investigations carried on in 

 these last two groups have already revealed numerous modifications 

 resulting from their habits and mode of life. It is reasonable to sup- 

 pose that these terraqueous copepods would also exhibit modifications 

 similar in their interpretation but differing in their details, and such we 

 find to be the case. 



The first of these modifications is shown in the restricted size of 

 the fully developed adult. In the other groups we find great variations 

 in size up to 200 mm in a few parasitic forms. Here there is great 

 uniformity in size, from a minimum of a quarter of a millimeter to 

 a maximum of half a millimeter in the sand dwellers, and a maximum 

 of slightly more than a millimeter in the mud dwellers. 



A second modification is one of shape ; it is evident that a linear 

 form will have greater freedom of motion under the restrictions of 

 the sand and mud than a rotund or corpulent form. The terraqueous 

 copepods all exhibit a more or less pronounced slenderness, the length 



