54 D. J. SCOURFIELD ON THE EPHIPPIUM OF BOSMINA. 



has taken place. The probabihty is that a line of hexagons has 

 been suppressed or rather completely modified to form the line of 

 weakness, and this view is borne out by the arrangement of the 

 shell markings shown in Fig. 7, where we seem to have a very 

 early stage of the process exhibited. The one long cell, which lies 

 exactly where a portion of the line of weakness is always developed, 

 has plainly been formed from the ordinary hexagonal markings, 

 because its edges show the characteristic zigzag arrangement, and 

 the crossbars are also just discernible, although almost obliterated. 

 It is somewhat strange that in the specimen from which this was 

 drawn there was no trace of any modification having commenced 

 for the production of the lateral bands and slits. It looks as if 

 the production of a line of weakness may be older in the history 

 of the development of these ephip^Dia than the formation of the 

 lateral chitinous bands. This may very well be so, because among 

 the Lynceidae, where there is in many cases scarcely any actual 

 modification of the shell, there is almost invariably a line of 

 weakness developed prior to the moulting of the ei^hippium. 



Having now seen how it is that the ventral portions of the 

 valves become so easily detached from the rest of the shell when 

 an ephippium is formed, we may very well ask ourselves why this 

 process should take place. As the phenomenon is so common 

 among the Cladocera there must be some fundamental necessity 

 for it. I think the answer to this question is undoubtedly to be 

 found in the fact that in the vast majority of cases it would 

 be quite impossible for the anterior margins of the valves to be 

 brought into contact if the ventral, and especially the anterior 

 ventral portions of the valves, owing to their very convex nature, 

 remained in position. In other words, it would be impossible for 

 the ephippium to form a closed covering for the egg. 



In addition to the outer protective covering which has just been 

 described, there are also some very delicate inner membranes 

 which surround the egg, as indicated in Fig. 1. They most 

 probably consist of the moulted inner layer of the shell valves, 

 and^ so far as can be seen, do not appear to have undergone any 

 special alteration. The resting egg itself — there is never more 

 than one in any ephippium — is very largely composed of small 

 globules of a dull greenish oily material. At the edges it is slightly 

 translucent, but elsewhere opaque. It can readily be distinguished 

 from a " summer " or parthenogenetic egg by its rather larger size 



