464 FRESH-WATER BIOLOGY 



tions, when they are really cuticular structures, are likely to appear 

 in some multiple of four. Since they occur on each side of the two 

 lateral lines, and, naturally, in a symmetrical way, the smallest 

 number possible is four. Two on each side of each lateral line 

 would make eight in all, a state of things exemplified in Iota. In 

 Diplogaster the number is about sixteen to thirty-two, and ap- 

 parently these numbers also prevail in some Dorylaimi. 



The various elements of the cuticula originate in certain cells in 

 the longitudinal fields, which early in the development of the em- 

 bryo become separated from the ectoblastomere group of cells. One 

 of the first two somablastomeres, the primary ectoblastomere, divides 

 and spreads systematically over the endoblastomeres. By further 

 divisions the primary ectoderm thus formed gives rise among other 

 things to the cuticula. The nuclei of the ectoblast cells destined 

 to form the cuticula of the embryo arrange themselves in longitudi- 

 nal lines. Increasing, and functioning from these lines they become 

 specially active at each moult in producing a new layer of cuticula. 

 At moulting time the activity of the cuticula-forming cells in the 

 lateral fields is indicated by an increase in the size of the nuclei 

 and the growth from them of excessively fine elements forming the 

 cuticula. The lateral fields appear to be the leading members of 

 this group of cuticula builders. This is in harmony with the greater 

 abundance and variety of the lateral cuticular structures. 



In the course of its development a nematode sheds its skin 

 about four times, and often appears to be about as active 

 during the moulting period as at other times. In some species 

 the changes that take place at the time of moulting are of a 

 striking character, reminiscent of the metamorphoses in other 

 groups, though no true metamorphosis takes place. Thus we have 

 in the last moult of the males in some species of Iota a marked alter- 

 ation, viz.j the loss of the oral spear. This so alters the appear- 

 ance of the male that one unfamiliar with the facts would not 

 class the adult males in the same genus as the females. 



During the moulting period the cuticula is thicker and looser, 

 sometimes very loose. The lining of the mouth and esophagus, 

 as well as that of the rectum, is shed at the same time as the outer 

 cuticula. At this time, therefore, the mouth parts take on unusual 



