THE PYCNOGONIDS. 21 



schizocoels. The surrounding mesoderm of these latter is irreg- 

 ular and quite different from that around the body-cavity proper. 

 The ectoderm over the appendages is thicker than elsewhere. 

 The ventral ganglia at this stage have in most cases the ventral 

 organs, although in this figure none are open and they have 

 closed rather early. The ectoblast of the dorsal surface is quite 

 thin and the mesoblast has not appeared in that part of the 

 animal. 



In Fig. 26 is a drawing of a cross-section through the head of 

 an embryo at a stage a little later than III, but not so late as Y. 

 The section passes through the brain above and the second pair 

 of ganglia below. Between the brain and the ventral ganglia 

 the stomodseum is seen. Its lumen is seen to be triangular in 

 outline, with its base turned upwards. On each side of the 

 stomodseum appear cross-sections of the first pair of gut-diverti- 

 cula. A single row of entoblast cells surrounds the central 

 mass of yolk. Scattered mesoblast cells are found around and 

 between these different structures, leaving here and there 

 openings between the cells. Such schizocoels are shown by S, S 

 in the figure. 



Going on to stage V we find that sections give little more 

 than was seen in the preceding figures. The prolongations of 

 the mid-gut into the legs have grown longer, and, as seen from 

 surface views, the first and second pairs of ganglia have fused. 



In stage VI the yolk mass continues to fill the central and 

 upper part of the embryo, but it now begins to decrease in 

 quantity. This may be due in part to the fact that part of it 

 becomes digested and built up into the tissue of the embryo, and 

 in part to its extension into the legs. 



A cross-section of an embryo at a stage a little older than YI, 

 in a plane of a pair of limbs, is shown in Fig. 27. The ventral 

 ganglia have increased enormously in size and show a large 

 amount of punkt-substance. In the middle of the section lies 

 the mid-gut with its contained yolk, which has decreased very 

 much as compared with preceding stages. 



In Fig. 28 is drawn a section of another embryo, at about 

 the same stage as the last, but cuts the embryo between a pair 

 of legs. Above the large ganglion is seen the digestive tract, 

 which is completely separated from the body-wall by a series of 



