NERVOUS SYSTEM OF PYCXOGONIDS 467 



The small ganglia of the proboscis are rather new structures, 

 but the large, represent the frontal and lateral head ganglia 

 of other forms. 



The develo]:>ment of the nervous system of pycnogonids 

 has been especially studied by ^Morgan. '91, and Meisenheimer, 

 '03, although a number of others have studied the general life 

 histories, or special stages. According to ]\Ieisenheimer, in 

 the embryos of Ammothea the early development of the ganglia 

 is much as in other arthropods, a longitudinal strip or germ band 

 enwTaps the yolk and paired thickenings of the ectoderm occur 

 which rei:)iesent cerebial and jiost-oral ganglia. I have not 

 followed these earliest stages in any of my material. At the 

 time that the free larva is liberated, there is seen a supraeso- 

 phageal ganglion and thiee i^airs of sub-intestinal centers such 

 as shown in figure 13 of a California foiin of the same genus. 

 The second of these two ganglia is comjiosed of two jiarts and 

 represents the second and thiid ])arts fused. This t>pe of larval 

 nervous system seems rather ty]ucal of this sort of larval form. 

 What the changes are from this to the adult are not exactly 

 known, but suggestions may l)e obtained fi'om tlie study of 

 other species. Morgan in Tanystylum gives some idea of the 

 gradual development of additional ganglia in the caudal region 

 as the larva with three i)airs of ajipendages add a fourth and a 

 fifth ])aii" successively and later a sixth pair. At this last period 

 \ho maximum lumibci' of ganglia is attained, this number be- 

 comes re(lu('(Hl with the growth of the seventh pair of appen- 

 dages and llie adult condition is reached. During the early 

 stages the aildition of extra gangha is probably not so much 

 from the backward growth of nervous tissue as from later de- 

 ^•elopments from the surface. In Palene the type of develop- 

 ment is different because of the large yolk mass. Separate 

 ganglia for the segments are developed, each of these has at an 

 early period an invagination or Central organ.' 



The species whose development I have especially studied 

 seems intermediate between the free living larval form and such 

 a continuous type of development as shown by Palene. This 

 genus Ano]ihlodactylus is more parasiMc during larval stages 



