THE VENTRAL ORGANS. 127 



layer of oval, anri an inner mass of more rounded elements. The 

 latter are much inclined to drop out in the sections, leaving a 

 surface layer of nuclei and thin protoplasmic strands passing 

 inwards. 



From what has been said as to their history it is obvious that 

 the ventral organs represent a portion of the ectoderm, from 

 which the central nervous system was constricted off. They 

 correspond in number with the segments, and are therefore 

 twenty in number in Capensis. 



1. The ventral organ of the first somite is probably, 

 as Kennel has suggested, represented by the cerebral grooves. 

 These become completely cut off from the surface ectoderm and 

 form the hollow appendages attached to the ventral side of the 

 brain of the adult (PI. X, figs. 2, 3, and No. 20, fig. 19, c, d). 

 The walls of these vesicles appear to consist of nervous tissue. 



2. The ventral organs of the jaws (PI. X, fig. 4, 

 V. 0. 1) come to lie in the buccal cavity on each side of the 

 mouth at the base of the jaws. Tbey differ from all the 

 posterior ventral organs in not coming into contact with one 

 another in the middle ventral line. They remain in the ecto- 

 derm, and appear to retain a connection with the posterior 

 lobe of the brain, or anterior part of the circumpharyngeal 

 commissure. 



3. The ventral organs of the oral papillae join one 

 another ventrally and become divided into two parts by the 

 lips — an anterior contained in the posterior region of the 

 buccal cavity, into which the salivary glands open ; and a pos- 

 terior part on the ventral side of the body just behind the 

 mouth. The intrabuccal part remains in connection with the 

 lateral nerve-cords (No. 20, fig. 14), and these two connections, 

 together with the interposed ventral organ, contribute what 

 Balfour has called the second commissure between the ventral 

 cords. 



The posterior part of this ventral organ behaves exactly as 

 do those aliout to be described. 



4. The two halves, of which each ventral organ of 

 the seventeen ambulatory legs at first consists, join one 



