No. 1.] THE DEVELOPMENT OF BALANOGLOSSUS. 39 
tudinally, or at right angles to the last series. The plane of 
the section passes through the opening where the blastocoel 
runs into the cavity formed by the inturned wall of the body- 
cavity. The pavement-like cells of the wall at this point, 
together with the foldings of the wall, are shown. The whole 
structure is suggestive of an excretory arrangement, but there 
is no direct evidence for such a view. The section passes 
through the anterior end of the proboscis vesicle, and the 
latter is cut twice, due to its antero-ventral wall being slightly 
bifurcated. 
In the next stage, shown in Fig. 4, Pl. I, the retrogressive 
changes have begun in the larval life. The general external 
phenomena have been described in a preceding section. From 
serial sections, we gather these additional facts. The ecto- 
derm is becoming thicker, as seen in Fig. 35. This is a 
portion of a longitudinal section, and shows the side of the 
body-wall and the two body-cavities (4.c.2 and 0.c.’). The 
ectoderm between the attachment of these is shorter than in 
the corresponding section of the preceding stage (Fig. 29). 
The body-walls are thicker, and contain more nuclei arranged 
in several layers (as to relative position, although the cells no 
doubt run through the ectoderm). The increase in the circum- 
oral ectoderm is relatively greater, obliterating the distinction 
between the two layers. 
The tentacles are becoming shorter. In some of the larvae 
that may be grouped around this “stage,” the tentacles are 
almost entirely absorbed, as in the larva figured in Fig. 4, Pl. I; 
in others they are larger, so that the degree of degeneration of 
the tentacles is not in itself a mark of the stage reached. In 
fact, and this may be said of almost any single organ, the 
organs have rarely reached the same relative degree of change. 
In Fig. 36 a cross-section of a tentacle in process of degen- 
eration is seen. The nuclei are arranged more irregularly, 
and many of them seem to be disappearing. Where the 
tentacle is attached at its base, as seen in Fig. 40, a very 
irregular connection with the rest of the ectoderm is found, 
and in the later stages the cavity of the tentacle disappears, 
