452 CHiETOGNATHA. 



l.st.g., are more widely separated from the forebrain than in most arthropods, 

 but otherwise they have a similar relation to the stomodaeum and forebrain. 

 They are united with each other by the same highly characteristic cross com- 

 missure, st.co., that passes over the anterior heemal surface of the stomodaeum, 

 and that forms such an important landmark in the brains of insects, arachnids, 

 and phyllopods. Moreover the lateral stomodaeal ganglia, judgmg from the 

 figures of Doncaster, appear to arise, as they do in the arthropods, independently 

 of the forebrain, from lateral thickenings, or evaginations of the walls of the 

 stomodaeum. See Chapter IV, p. 60. 



Cephalic Sense Organs. — The cephalic sense organs consist of two compound 

 lateral eyes. I.e., a parietal eye, p.e., and the frontal or olfactory organs ("corona 

 ciliata"), cox. These important sense organs have the same general arrange- 

 ment, innervation and structure that is so characteristic of the cephalic sense 

 organs of all primitive arthropods. Chapter VIII, p. 125. 



The lateral eyes, I.e., consist of three retinulae, or three small optic cups, 

 united to form a single organ. The rhabdoms are terminal and nearly upright, 

 that is they are directed outward toward the optical center of each cup and 

 toward a centrally located refractive body consisting of three sharply defined 

 segments These eyes represent the compound lateral eyes of arthropods in a 

 rudimentary or elementary condition. 



The parietal eye ("Fossetta Retrocerebrali " of Grassi), including its two 

 appendages, is a tri-lobed sac, lying on the posterior median margin of the fore- 

 brain. Except for the absence of the black pigment that ordinarily makes the 

 parietal eye so conspicuous, it is very similar to the parietal ocellus of the nauplius 

 of Apus, Branchipus, or Lernaea. 



The eye sac consists of a median portion opening freely to the exterior, and 

 representing the unpaired ocellus with its short epiphysis and pore, while the two 

 paired ocelli lie in the diverging blind sacs on either side. The globules and 

 glistening granules described by Grassi, that are contained in these sacs, no doubt 

 represent the vesicular retinal cells filled with the white pigment so commonly 

 present in the degenerate parietal eye, e.g., Limulus and Petromyzon. The eye 

 lies directly on the posterior surface of the brain, as in Apus, and appears to be 

 connected with it by short but indistinct nerves. 



The Olfactory Organ. — Between the parietal and lateral eyes are two large 

 nerves, co.n., one on either side, that represent the frontal-organ nerves, or the 

 olfactory nerves of arachnids, phyllopods and entomostraca. They terminate 

 diffusely, that is, in widely distributed, subcutaneous branches, as is characteristic 

 of these nerves in the above mentioned forms, in a large sensory area enclosed by 

 a prominent ciliated groove, " corona ciliata" of Grassi. This area, the surround- 

 ing ciliated groove, and the appertaining nerves (two pairs ?) represent the frontal 

 or olfactory organs of the crania ta. See Chapter X, p. 160. 



The Ventral Ganglion. — The ventral ganglion is relatively large and complex. 

 Its minute anatomy has not been carefully described, but it appears to have the 



