appendages and their nerve ganglia may be seen on the surface. Above 

 and anterior to the stomodaium are the thickenings of epiblast to form the 

 brain. On each side of the invagination the first pair of appendages arises. 

 Behind tlie stomodiEnm five pairs of large ganglia appear, and on each 

 side of the three posterior pairs are formed the fourth, fifth and sixth pairs 

 of appendages. * 



At the " lower " pole of the embrvo (at this time the dorsal and poste- 

 rior) the nuclei are slowly multiplying and cover the surface with thin 

 protoplasm. Now the embryo lengthens in the antero-posterior direction, 

 the appendages and their ganglia become more conspicuous, and there is 

 seen a slight invaginalion in the centre of each ventral ganglion. There are five 

 pairs of these invaginations corresponding with the same number of gan- 

 glia, and I shall call them the ventral orgatis. 



Fir,. 8. 



Cross sections of embryos show distinctly a wide ingrowth of the surface 

 epithelium into the centre of each ganglion. The cells of its walls ai'e 

 rather high, with a clear outer portion, and with large nuclei. Fig. 8 

 shows such a cross section. These organs close on their outer surface, 

 and there remains in the centre of each ganglion a cavity, which is rather 

 longer than wide from side to side, and persists till quite late in- embryonic 

 life. 



I need only refer to similar (?) invaginations in Peripatus, and more_ 

 especially to a section of a pair of these organs figured by Sedgwick for the 

 ventral organs of the jaws of Peripatus (Studies Morph. Lab., Cambridge, 

 Vol. IV, Pt. I, Plate 10, Fig. 4), which easily suggests a comparison with 

 an early stage in the development of the ventral organ of Pallene. Although 

 I have looked very carefully I have not satisfied myself as yet as to whether 

 or not there are any invaginations for the brain of the ^ea Spiders. The 

 appendages grow in length, and into each there is pushed an outgrowth 

 from the mesenteron. These outgrowths contain yolk, and this is covered 

 by a layer of endodermal cells. Along the ventral half of the embryo there 

 are scattered mesoderm cells, and these extend into the appendages between 

 the endoderm and ectoderm cells. 



In Pallene the second pair of appendages, which are found in other pyc- 

 nogonids, never appear, and seem to have been completely dropped from 

 the ontogeny. The third pair of appendages (the egg carriers of the male) 

 appear at the time when the young is about to leave the parent. Two pairs 

 of nerve ganglia develop for these appendages, and each ganglion contains 

 a ventral organ. Later the two pairs form a single pair with four ventral 

 organs. The presence of these ventral organs is conclusive proof as to the 



erroneousness of Schimkewitsch's hypothesis, that the third pair of append- 

 ages are outgrowths of the second pair. The fourth, fifth and sixth pairs 

 of appendages appear (as already given) simultaneously and quite early. 

 The seventh appears a little before the embryos leave the parent. Like- 

 wise do the ganglia of the seventh appear quite late. There is in addi- 

 tion one pair of ganglia for the rudimentary abdomen. The yolk mass 

 becomes smaller as it is eaten by the endoderm cells, and exceptionally a 

 wandering endoderm cell may be found in the mass of yolk. As the yolk 

 disappears a number of schizocoels appear in the mesoderm between the 

 body walls and the digestive tract. Dorsally the heart appears as a simple 

 tube. The stomoda;um communicates with the mesenteron and the proc- 

 todjeum forms very late— at the time when the seventh pair of appendages 

 appear. Soon the embryo leaves the parent and no doubt crawls off" among 

 the sea weeds and hydroids to shift for itself. 



