159] LIFE HISTORY OF GORDIUS AND PARAGORDIUS—MA Y 39 



cells in cross section with the nucleus lying close to the inner edge. As 

 development goes on the cells become more and more flattened and 

 elongated (Figs. 112, 117). When viewed from the edge the cells take on 

 the appearance of very much elongated spindles with the two ends running 

 out into fine points. From the side they appear as long blades with one 

 edge straight and the other rounded at the ends (Fig. 126). When in posi- 

 tion the blades are placed with the straight edge against the hypoderm and 

 the nucleus is located in the middle of the opposite edge. The ends can 

 easily be detected in cross section lying in the outer half of the muscle 

 layer (Fig. 112). The nucleus, at first nearly round, later comes to be a 

 very much elongated, flattened, oval body, lying either at the inner edge 

 or near the inner edge of the cell, and occupying nearly the whole diameter 

 of the cell at that point (Fig. 107). 



The cytoplasm of the cell at first does not appear different from that 

 of other cells but later there is formed a deeply staining granular substance 

 extending from the nucleus to the outer edge of the cell; this substance 

 finally forms longitudinal fibrils which arrange themselves in a continuous 

 layer around the inner, spongy cytoplasm and the nucleus (Figs. 43, 105, 

 112, 124, 126). The fibrils are not of homogeneous structure, but are 

 composed of serially arranged granules. 



The cells at first are contiguous, but at the time the heavy walls appear 

 in the parenchyma a substance having the same appearance and staining 

 reactions as those walls surrounds each cell, so that the cells become 

 separated from each other, from the hypoderm and from the parenchyma 

 (Fig. 116). At the ends of the body the muscles gradually lose their 

 characteristic structure and pass over into the parenchyma. 



At the time of the discharge of the reproductive products the muscles 

 begin to disintegrate slowly from the inner edge. In some specimens 

 sectioned this process had consumed nearly the whole muscle cells (Figs. 

 46, 124). 



The cloacal musculature of the male consists of radiating fibres around 

 the cloaca and circular fibres surrounding the sperm ducts just before they 

 enter the cloaca (Figs. 96, 97). In the early stages the cells are not 

 differentiated from parenchyma cells, but later they become very much 

 elongated and in the adults lack the heavy cell walls that are found in the 

 parenchyma. The radiating fibers arise from the dorsal and lateral walls 

 of the cloaca and extend for the main part in a dorso-lateral direction. 



The cloacal musculature of the female consists of a heavy circular muscle 

 forming the constriction between the cloaca and the sperm receptacle 

 (Fig. 94) and weaker circular muscles around the oviducts. The fibers 

 are similar to those of the male cloacal musculature. There is also present 

 a heavy group of circular fibers around the posterior end of the cloaca, 

 and a sheet of longitudinal fibers surrounding the glandular part of the 



