226 WILSON. [Vot. II. 
that each new mesentery appears along the dorsal meridian 
between the two last formed, and in such a way that the suc- 
cessive mesenteries come to lie on opposite sides of the dorso- 
ventral plane. The directive pair, D.JZ, corresponds to the 
directive pair which the Hertwigs have figured (Taf. I., Fig. 8). 
These authors state, however, that this pair is the longest of 
all, extending to the anal pore. In the young specimens I have 
studied, this pair disappears some distance above the pore, and 
is exceeded in length by two or three. mesenteries on each side 
of the dorso-ventral plane. Von Heider (11) made the discovery, 
which the Hertwigs confirmed, that in the adult there is a much 
shorter pair enclosed between the two directive mesenteries. 
This short pair only extends the length of the cesophagus. It 
must appear in stages later than Fig. 49, so that besides the 
formation of mesenteries which takes place dorsally, at least 
one pair is formed ventrally. 
Filaments were found only on the complete mesenteries. 
Fig. 47 is from a section just below the cesophagus. The mes- 
entery is composed of very large cells with immense vacuoles. 
At the edge of the mesentery, just beneath the filament, the 
tissue is becoming more compact. This process has gone much 
farther in Fig. 48 (from the same mesentery at a lower level). 
The whole mesentery, as may be seen by comparing my fig- 
ures with Hertwigs’ Taf. VIII., gradually suffers this change as 
the animal becomes adult. In Fig. 47 the filament has a cen- 
tral body provided with gland and nettle cells, and two lateral 
ciliated tracts 7, composed of supporting cells, which are grad- 
ually assuming the shape of distinct lobes owing to the subjacent 
growth of the jelly. The central body passes directly into the 
epithelium of the cesophagus, the lateral tracts becoming indis- 
tinguishable from the body of the filament just before the transi- 
tion. This figure is but slightly different from the section of 
the adult filament given by the Hertwigs. At a lower level, 
Fig. 48, the ciliated tracts are no longer found. On many mes- 
enteries the filament has almost no connection with the endo. 
derm, as in the figure. In such figures as Fig. 48, it need hardly 
be said that violent contraction has had much to do with pro- 
ducing such complete separation of filament and mesentery; 
but this only shows how loose the connection is in early life. 
On other mesenteries the filament is continuous at its sides 
