THE PYCNOGONIDS. 13 



the periphery of the yolk appear the six pairs of appendages, 

 and behind, the thickened posterior mass of the embryo. 



Returning to the ventral view of the embryo (Fig. Ill), there 

 is seen a most interesting structure in the center of each ganglia. 

 Each is an invagination of the surface into a ganglion, and these 

 invaginations are elliptical in outline, with the long axis corres- 

 ponding to that of the embryo. These structures I shall call the 

 Ventral Organs, and, by means of serial sections, we shall later 

 study them in more detail. In the next stage, shown by Fig. V, 

 the embryo is more oval in outline. The appendages have also 

 elongated and become more bent. The posterior pair have 

 begun to grow forward between the more anterior pairs. The 

 most important change is in the fusion of the first two pairs of 

 ganglia, which now appear as one pair of rather large ganglia. 

 This pair of ganglia still shows its double structure by the pres- 

 ence of two pairs of ventral organs. The third pair of ganglia 

 also shows a pair of ventral organs, but the more posterior ganglia 

 are covered by the posterior appendages, so that these ganglia 

 cannot be seen from the surface. 



After this stage is passed the appendages grow enormously in 

 length, and the embryo becomes flattened from side to side. A 

 figure of an embryo at this stage is shown in side view in Fig. VI. 

 The first appendage is chelate, and has a joint near its base, and 

 one also sees it has moved more dorsally to the proboscis. Beyond 

 and beneath this appendage appears the proboscis, which has 

 much elongated. Near the base of the appendage is seen part 

 of the brain. This figure also shows that the yolk mass is con- 

 tinuous along the center of each ambulatory limb. The three 

 pairs of legs are much bent, and each ends in a spine-like process. 

 The body is seen to end behind in a knob-like projection. The 

 ventral ganglia are shown between the bases of the legs, and the 

 height of each at this stage is about equal to its length. In older 

 embryos the yolk begins to disappear and the sides of the embryo 

 to become thicker. After this the embryo lengthens a great deal, 

 the appendages grow much longer and become segmented. 

 Another pair of appendages appears behind the third pair of 

 walking legs, and the knob-like projection at the end of the 

 embryo is pushed more dorsally, to form the rudimentary abdo- 

 men. Four pairs of eye spots appear over the posterior end of 

 the brain. 



