CHANGES OF FORM OF THE LARVA. 21 



stages, I will take up the development of the water-tubes at the point to 

 which we had traced them (PI. II. Fig. 28) in the larvae of A. berylinus. 

 After the ends of the water-tubes have extended beyond the oral opening 

 (PL III. Fig. 4), the tubes increase rapidly in diameter (PI. III. Figs. 6, 8, 

 w, w'), bending at the same time towards the longitudinal axis (PI. III. 

 Figs. 4, 5, 6, 8, 10, iv, iv), the other extremity of the tubes creeping round 

 the stomach until they touch, but without uniting (PI. III. Figs. 6, 8, 10, 

 IV, tv). The tubes at the oral extremity bend towards each other (PI. III. 

 Fig. 4), come in contact (PI. III. Fig. 6), and, soon after, a communication 

 is made, the water-system assuming the shape of an elliptical ring (PI. III. 

 Fig. 6, ivw') ; and the water which enters into the right tube through the 

 dorsal pore (PI. III. Figs. 2, 5, 7, b) passes into the other branch on the 

 opposite side of the stomach, through the fork at the oral extremity, and 

 not round the stomach, where the water-tubes simply touch, but do not 

 communicate. The small tube leading from the dorsal pore to the main 

 branch of the water-system widens and becomes funnel-shaped as it ap- 

 proaches the main tube. The dorsal pox-e is cut obliquely across the end 

 of this small tube, giving it an elliptical shape. By the time the two 

 branches of the water-system have joined (PI. III. Fig. 6) at the oral ex- 

 tremity of the larva, it has assumed an entirely different outline from 

 au}^ we have met with in the former species. The anal extremity is 

 very much flattened, the corners of the anal plastron project slightly be- 

 yond the general outline, the indentations have become very distinct, 

 the oral plastron has grown rectangular with rounded angles and concave 

 sides, the oral triangular opening leads into a deep pouch. The sides of 

 the body are marked by three strong indentations (PI. III. Fig. 8). The 

 oral extremity of the water-system changes rapidly from a rounded to a 

 pointed outline (PI. III. Fig. 8, ivtv) ; it advances more and more towards 

 the oral extremity. In proportion as the dorsal region projects beyond 

 the oral plastron, the water-system extends into this projection, sending 

 off, at the same time, two branches leading into small appendages (PI. 

 III. Figs. 10, 11,/,/), (only developed in more advanced larvce), which 

 have, in the adult larvae, a peculiar structure (PI. IV^. Figs. 4, 5, 6). 



Changes of Form of the Larva. — The prominent changes now going on 

 are only changes of degree. The larva has completely lost its cylindrical 

 shape, and even the pear-shaped form it assumed afterwards ; it has be- 

 come rectangular, with deep indentations, gradually assuming the char- 



