THE NERVOUS SYSTEM AND THE YEXTRAL ORGAN-. 191 



another on the ventral side of the cephalic segment (Fig. 96 B, 

 which have arisen, like the ventral organs of the trunk, by the 

 splitting up of the ectodermal layer into an outer and an inner 

 portion, and the subsequent invagination of the former. These 

 depressions, -which at first are quite open, but later close almost 

 completely, can be recognised even in surface view, first as pits, and 

 later as irregular slits on the ventral surface of the cephalic lobes 

 (Figs. 93, p. 18-1, and 94 B, p. 185). At a later stage the ventral 

 organs close completely and lose their connection with the epidermis. 

 As the two vesicles sink down deep into the mass of the brain 

 (Fig. 96 B) and thus become closely connected with this latter, it is 

 clear that, when the brain becomes small in comparison to the head 

 and shifts to the dorsal side of the latter, the vesicles follow the 

 brain, and remain connected with it in the form of a thick-walled 

 vesicle, the so-called brain-appendage of Peripcdu*. The ventral 

 organ of the cephalic segment, if, indeed, this vesicle is to be 

 considered as such, would be distinguished from those of the trunk 

 by the complete loss of its connection with the epidermis. 



The significance of the ventral organs has until now remained obscure. Their 

 great development in the early part of embryonic life, and their reduction in 

 the adult, indicates that they are organs which were more highly developed in 

 the ancestors of Peripatus. From their position it might be concluded that 

 perhaps the greater part of the ventral surface, by means of its strong ciliation, 

 functioned for locomotion, like the ventral ciliated area of the Annelida. The 

 connection of the ventral organs with the nervous system is not surprising, 

 considering the origin of the latter out of these ectodermal masses. It is 

 possible that during ontogeny the ventral organs may be concerned in supply- 

 ing the cell-material for the development of the ventral chain of ganglia, 

 v. Kexxel's statement that the gradually diminishing cell-mass of the ventral 

 organs is used in the further development of the epidermis seems in keeping 

 with the original connection of these organs with the ectoderm, especially as, 

 with the exception of the ventral organ of the cephalic segment, the greater 

 j>art of each organ retains this connection. The similarity between these 

 cephalic ventral organs and the "cephalic pits" of the Arachnida, which are 

 in the closest connection with the nervous system (pp. 12 and 53), is very 

 striking. Fig. 96 B shows how closely the "ventral organs" of the head of 

 Peripatus become applied to the rudiment of the brain, and comparison of Figs. 

 93 and 94 B, with Fig. i C, p. 6, Fig. 7, p. 10, and Fig. 28, p. 52, shows that a 

 marked agreement exists even in the external position of the depressions in the 

 two groups. In the present state of our knowledge, however, we are not 

 justified in carrying this comparison further.* 



The Nervous System. When the rudiments of the two longi- 

 tudinal nerve-trunks first separated from those of the ventral organs, 



* [Willey (App. to Lit. on Onychophora, Xo. II.) finds what he believes to 

 be ventral organs persisting in the adult on the anal segment. — En.] 



