NERVOUS SYSTEM OF PYCNOGONIDS 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 development 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 Meisenheimer, in 

 the embryos of Ammothea the early development of the ganglia 

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

 enwraps the yolk and paired thickenings of the ectoderm occur 

 which represent cerebral and post-oral ganglia. I have not 

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

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

 phageal ganglion and three pairs of sub-intestinal centers such 

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

 The second of these two ganglia is composed of two parts and 

 represents the second and third parts fused. This tyi3e of larval 

 nervous system seems rather typical of this sort of larval form. 

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

 known, but suggestions may be obtained from the 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 pairs of appendages add a fourth and a 

 fifth pair successively and later a sixth pair. At this last period 

 the maximum number of ganglia is attained, this number be- 

 comes reduced with the growth of the seventh pair of appen- 

 dages and the adult condition is reached. During the early 

 stages the addition of extra ganglia is probably not so much 

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

 velopments from the surface. In Palene the tyiDe 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 'ventral 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 Anophlodactylus is more parasitic during larval stages 



