472 INVERTEBBATE MORPHOLOGY. 



the type consists of a siipraoesophageal ganglionic mass, con- 

 nected by circumoesophageal commissures with a chain of 

 ventral ganglia, a pair of ganglia corresponding typically with 

 each segment. In the Insecta (Fig. 228) more especially, 

 however, considerable concentration occurs, a number of the 

 postoral ganglia, or, in some cases, all of them, fusing to a 

 single mass. A well-developed stomatogastric or sympathetic 

 nervous system occurs in all forms, arising from the supra- 

 cesophageal ganglionic mass by two trunks, which unite to 

 form a single nerve, passing to the digestive tract, and in 

 some cases provided with ganglionic enlargements both 

 paired and unpaired. 



8ense-organs of various kinds are well developed in the 

 Tracheata, with the exception of Peripatus, in which the 

 only definite organs of special sense are the eyes. In other 

 forms the antennae and other portions of the body are pro- 

 vided with hairs connected with nerves and serving as tactile 

 organs, and setae situated upon the mouth-parts and associated 

 with peculiar nerve-endings have been supposed to represent 

 organs of taste, and others again, on the antennae, olfactory 

 organs. Eyes are very generally present. In Peripatus and 

 most Myriapoda simple eyes or ocelli are alone present ; in 

 Peripatus they resemble closely in structure the eyes of the 

 Annelids or Mollusca (e.g. Haliotis, see Fig. 127), but in the 

 Myriapods and Insects they are usually more complicated. 

 Thus in a young larva of Acilius (Fig. 216, A), a water- 

 beetle, the chitin is thickened to form a cornea (Q which lies 

 over a depression of the hj^podermis, the cells at the bottom 

 of which are modified to form a retina, each being continuous 

 at its inner end with the optic nerve ();), while at its outer 

 end it bears a layer of chitin (r). The cells of the lip of the 

 depression have converged together so as to meet beneath the 

 cornea, which is indeed formed by these cells, and a cavitv is 

 thus enclosed into which there protrude from amono- the 

 retinal cells large cells {mgc) with chitin deposited on their 

 adjacent surfaces. In a later stage (Fig. 216, B) the lips of 

 the depression have united, a continuous corneal hypodermis 

 {vh) being thus produced ; pigment has been deposited in the 

 lateral cells, and the retinal cells, pigmented near their outer 



