414 



GENERAL CONSIDERATIONS ON 



be regarded as specially primitive, and is to be found taking place in 

 a somewhat similar manner in all the various groups. 



AVhile the formation of the chain of ganglia takes place, as a rule, 

 by a process of delamination from temporary ectodermal grooves 

 which afterwards vanish, permanent invaginations occur which take 

 part in the formation of the brain, leading no doubt chiefly to the 

 formation of the optic ganglia. The appearance of these more or less 

 extensive depressions, known as cephalic pits, is specially character- 

 istic of the various divisions of the Arthropoda (Perijmtus, the 

 Myriopoda, the Insecta, Limulus, the Arachnida). In PerijKitus, 

 indeed, another significance has been attributed to these depressions, 

 and it is doubtful whether they participate in the formation of the 

 brain. The depressions in the cephalic region in Peripatus corres- 

 pond to similar pit-like invaginations arranged in pairs which 



recur in each of 

 ,.(. the trunk -seg- 



ments. The na- 

 ture of these 

 remarkable struc- 

 tures, which are 

 very characteris- 

 tic of Peripatus, 

 has not yet been 

 established, but 

 similar depres- 

 sions have been 

 described in the 

 Myriopoda and 

 the Pantopoda. 

 The development of the eyes may be closely connected with the 

 cephalic pits just mentioned which, as it appears, chiefly give rise 

 to the formation of the optic ganglia (Scorpiones, Araneae). How- 

 ever much the permanent Arthropod eyes vary with regard to structure, 

 they may, in the first instance, be traced back to pit-like depressions 

 of the ectoderm, and in explaining them we must start from such 

 simple eyes as those occurring in the larvae of Insects and in many 

 Myriopoda. This simplest form of Arthropodan eye, the ocellus 

 (Fig. 195), consists of a depression of the hypodermis, the cells of 

 which have become differentiated into the so-called vitreous body 

 (gl), and retinal cells (rt), secreting rods. The unilaminar character 

 of the hypodermis has, however, been retained in this simple eye, so 



Fig. 195. — Section through the ocellus of a Dyliscus larva (after 

 Grenacher). ch, chitinous covering of the body ; gl, vitreous 

 hody ; hyp, hypodermis ; I, lens; n, optic nerve; rt, retina; st, 

 rods. 



