240 E. W. MACBKIDB. 



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 oesophagus 

 a. oe is also seen, and we notice the relation of the oral coelom to 

 it, and we may remark that the larval oesophagus is by this time 

 disrupted from the gut. Fig. 49 shows that dorsally the hydro- 

 coele is completely shut off from the anterior ccelom, and shows 

 that the oral coelom dorsally opens into the left poste- 

 rior cffilom. Fig. 48 shows that the opening of the oral coelom 

 is in close relation to a process of the left posterior coelom extend- 

 ing over to the right, dorsal to the gut. This is the right dorsal 

 horn (see p. 233 for the ventral horn) of the left posterior coelom, 

 and it is marked l"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 (PI. XIV, fig. 61). The opening of the oral 

 coelom 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 larvae, 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 perihsemal 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 (PI. XXII, 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 into the septum separating these two ring-canals. 

 Into the inner of the circular canals a vertical canal opens 



