No. 3.] GROWTH AND METAMORPHOSIS OF TORN ARIA. 413 



cavity of the heart is correspondingly enlarged. Fig. 22 (e) 

 shows the opening of the heart into the general blastoccel 

 space. The blood which fills the heart is bounded by the 

 gelatinous-like substance filling the cavity of the blastocoel. 



In older stages the proboscis vesicle applies itself very closely 

 to the enterocoel, and extends farther upwards, i.e. in the direc- 

 tion of the thickened ring. A series of cross-sections of an 

 older stage are shown in the Figs. 23, 24, 25, 26, and their 

 plane with reference to the enterocoel is shown by d, e,f, g, of 

 Fig. 18. Fig. 23 (d) shows the opening of the heart (h) out into 

 the blastoccel cavity. The lower wall is applied closely to the 

 stomach wall (w). A narrow partition (v) runs across the enter- 

 ocoel, and separates the right and left horns. Fig. 24 (e) is at 

 a higher level, and cuts through the heart (h) where it projects 

 into the cavity of the enterocoel. It is bounded by the walls of 

 the enterocoel on two sides, and posteriorly by the wall of the 

 vesicle (v). Fig. 25 shows the heart (h) hanging in its cavity of 

 the enterocoel. The walls of the enterocoel are thickened over 

 the blood-vessel, and are very irregular in outline. Posteriorly 

 (above) is seen the small lumen of the vesicle (v). Fig. 26 (g) 

 is from a section through the ring of tissue, and shows to what 

 extent the walls have thickened in this period. 



Returning to the larva shown in Fig. 5, we find four new 

 enteroccels are in process of formation, making five in all. I shall 

 speak of these as the second and third pairs of body cavities, or 

 simply as the paired body cavities, as distinguished from the ante- 

 rior unpaired enterocoel. The third pair appear slightly before the 

 second although their appearance is almost simultaneous. The 

 second pair arise at two opposite points on the side walls of the 

 posterior part of the stomach at the point marked be 2 in Fig. 5. 

 At these points there is a proliferation of cells (be 2 ), as shown 

 by the series of Figs. 27, 28, 29, PL XXV. So far as I can judge 

 from the sections obtained, it appears that we have here a pro- 

 cess of proliferation of the walls of the stomach, so that the 

 wall at this point, by division of its cells, becomes two-layered. 

 There is no evidence of evagination or pouching, although it 

 could not positively be denied that some of the cells may pull 

 out from the wall and come to lie on its outside. The outer cells 

 are more granular, and stain more deeply with hasmatoxylin 

 than the cells of the wall. In other cases I have seen a few 



