184 BULLETIN : MUSEUM OF COMPA.RATIVE ZOOLOGY. 



are cut perpendicular to the long axis of the antenna. No section plane 

 passes through the external orifice of the duct, but the orifice is near the 

 plane of the section shown in Figure 46. The opening to the exterior, 

 it will be observed, is not at the middle of the ventral face of the ap- 

 pendage, but more toward the median side. The communication be- 

 tween the lumina of the endsac and ectodermic sac does not appear, as 

 it is small and its axis is inclined to the plane of the section (cf. Figure 

 33), but it lies between Figure 47 and the next following section. Its 

 position is indicated in Figure 47 by a. Tt will be noticed that 

 the endsac lies partly ventral and partly lateral to the ectodermic sac 

 (Figures 44 to 48), and that its free margin extends dorsally, while 

 the ectodermic sac extends ventrally. This is the beginning of a pro- 

 cess of growth that is destined to reverse the relative positions of the 

 two parts as found in the embryonic stages, where the endsac was ven- 

 tral and the ectodermic sac dorsal (cf. Figs. 26, 27, 28, Plate 3). 



Finally, Figure 49 (Plate 6) represents the dorsal face of a frontal 

 section of the right gland, — the tenth, counting from the dorsal foce of 

 the gland, in a series of eighteen, each 10 micra thick, — in which is 

 seen the lateral position of the endsac and the communication between 

 the endsac and the ectodermic sac at a. The passage is situated at 

 the lateral and proximal part of the endsac. 



The evidence from sections in the three planes shows that the endsac 

 lies ventral and lateral to the ectodermic sac ; that the ectodermic sac 

 is elongated in the axis of the antenna, and gives off the duct to the ex- 

 terior from its ventral face well over on its median border ; and that the 

 gland lies almost wholly within the appendage and not in the cavity of 

 the body proper. 



The greatest diameter of the gland measured in each of the three axes 

 gave as a result of the average ^ of several cases the following : — 



Maximum proximo-distal axis, 0.3 mm. 

 " dorso-ventral axis, 0.2 " 



" latero-median axis, 0.2 " 



The average length of the first larva is 7.8 mm. from tip of rostrum to 

 tip of telson. 



1 The extremes of mensurement vary from these moan averntres 20 ^r in some 

 cases, wliich is probably duo to slifilit differences in the age of larvae, or to indi- 

 viduul differences independent of age. 



