3808 E. W. MACBRIDE. 
posterior end of the body, and we can also make out an arm 
rudiment, which at this stage is a mere protrusion of ectoderm 
filled with mesenchyme cells; it forms the extreme posterior 
end of the section. The rudiment of the adult cesophagus 
a. ce is also seen, and we notice the relation of the oral ccelom to 
it, and we may remark that the larval esophagus is by this time 
disrupted from the gut. Fig. 49 shows that dorsally the hydro- 
coele is completely shut off from the anterior coelom, and shows 
that the oral celom dorsally opens into the left poste- 
rior celom. Fig. 48 shows that the opening of the oral ccelom 
is in close relation to a process of the left posterior ccelom extend- 
ing over to the right, dorsal to the gut. Thisis the right dorsal 
horn (see p. 351 for the ventral horn) of the left posterior coelom, 
and it is marked /’p”c” in all the figures. In later stages it 
extends ventrally for a short way, insinuating itself between 
the gut and the septum dividing the anterior coelom from the 
left posterior one (Pl. 21, fig. 61). The opening of the oral 
celom is later shifted so as to be connected only with the right 
dorsal horn, and hence it came to pass that Ludwig regarded 
oral coelom and right dorsal horn of the left coelom as one 
structure, and described the oral ccelom as the oral blood-ring 
and the dorsal horn as the “ heart.” In common with all other 
growing spaces in the larve, this right dorsal horn has at its 
growing tip an epithelial thickening, and it was this which in 
my preliminary account I mistook for the rudiment of the 
“heart.” 
Figs. 51—53, taken from a slightly older larva, show the 
appearance of the rudiments of the perihemal spaces. It may 
be useful to refresh our memory of the arrangement of these 
spaces in the adult; this the annexed woodcut is intended to 
do. They are usually described as consisting of a canal situated 
just aboral to each radial nerve, and divided by a longitudinal 
septum (Pl. 29, fig. 155). These radial canals open into a 
circular canal surrounding the mouth, inside which is another 
inner ring-canal. The longitudinal septa of the radial canals 
are inserted in the septum separating these two ring-canals. 
Into the inner of the circular canals a vertical canal opens 
