498 Cresswell Sbearer 



the lumen of one of tlie diverticuhi is sliown cut in tlie sectìon. 

 As the larvae grow the ends of the divertieuUx are drawn out luto 

 processes, and these are the rudimentaiy nephrìdia (figs. 8 and IO). By 

 the time the rudimeuts of the ncphridia have made their appearance, 

 the originai cavity of the pit with its median opening on the exteiior 

 has beeome much reduccd in sizc. First this median opening 

 (figs. 27, 37, 38) becomes partially obliteratcd, and then divides into 

 two portions, which remain as the opeuiugs of the two nephridia. 

 The rapid growth of the aual end of tlie larva separates the openings 

 so that they come to take lateral positions, one on each side of the 

 anal papilla (figs. 43 and 50). This obliteration of the pit is caused 

 by the formation of the anus and rectum, which forms at tliis timo, 

 as au outgiowth from the stomach or gut. This forces the pit in a 

 ventral direction, and at the sanie time partially closes it up. 



Once the rectum is formed, the uephridial pit rapidly disappears, 

 and nothing remains but these diverticuhi having separate openings, 

 one on either side of the anus. A depression, however, for some 

 time marks the former position of the pit, on the external surface, 

 below the anal papilla. The rudiments of the nephridia 

 rapidly leugthen out into bulbshaped tubes (pi. 31 figs. 10 and 15). 

 The nuclei in the walls of these tubes are large and readily 

 distinguishable , and largest at the growiug end of the bulb. As 

 the tubes lengthen, they lose their bulb-like character, and assume 

 the typical appearance of the nephridia, as in fig. 1*2. Figs. 11, 16, 

 25 and 29 represent various stages in this change. The solenocytes 

 develop as direct outgrowths of the tube wall, and are at first large 

 and irregulär. They soon aggregate at the distai end of the tube, 

 where they take on the appearance they present in the fully formed 

 ncphridium. Fig. 28 represents the fully developed larvai eondition. 

 The solenocytes are therefore especially developed cells of the 

 nephridial wall, and so with the nephridium are of ectodormic origin. 

 In the early stages of their development they often appear to throw 

 out irregulär protoplasmic processes as in fig. 11. These i)rocesses 

 have, no doubt, given rise to their being mistakeu for meso- 

 dermic cells of the blastocoel in which the nephridia lie free at 

 this time. In the later stages, as in figs. 12 and 28, the solenocytes 

 are more regulär in form, and in the liviug eondition look not unlike 

 a small cluster of grapes at the end of the long nephridial tube (fig. 55). 



The lumen of the nephridium forms early, and is seen 

 almost from the first as a clear space in the ceutre of the mass of 



