398 ARTHROPODA. 



iris, cornea, vitreous body, and lens), as is the nervous system, 

 which has, in addition to the usual centres, optic, sympathetic, 

 and stellate ganglia. 



40. The eggs have a discoidal segmentation. 



41. The Cephalopoda are divided into Tetrabranchia and 

 Dibranchia. 



42. The Tetrabranchia (extinct save for Nautilus) have four 

 gills, a chambered shell, primitive eyes, and finger-like cephalic 

 lobes in place of tentacles. 



43. The Dibranchia have two gills, eight or ten tentacles with 

 suckers, and the shell is reduced or absent. 



PHYLUM VII. ARTHROPODA. 



Under the term Arthropoda are included the spiders, crabs, 

 insects, and myriapods, which, together with the annelids, were 

 united by Cuvier to form his sub-kingdom Articulata. Annelids 

 and arthropods agree in many features. They are, as the term 

 articulates implies, segmented animals, and they differ from the 

 vertebrates, which are also segmented, in the extension of the seg- 

 mentation, the ringing of the body, to the external surface. The 

 boundaries between the successive segments, which cannot be rec- 

 ognized in the skin of the fish or other vertebrate, are marked in 

 the articulates by a constriction of the body wall, whence the old 

 names evro^cx, Insecta, applied to these forms. The articulates 

 are further characterized by a ladder-like nervous system in which 

 the brain, present in most invertebrates, is supplemented by a 

 ventral chain composed of ganglia metamerically arranged. The 

 most evident distinctions between the annelids and the arthropods 

 are (1) the character of the segmentation and (2) the presence of 

 jointed appendages. 



In superficial appearance the lines between the segments are 

 constricted more deeply in the arthropods than in the annelids. 

 The cause of this lies in the character of the integument (fig. 25, /), 

 which is developed as a hard armor, in which two layers are rec- 

 ognizable, the epidermis (often called hypodermis) and the chitin- 

 ous layer. The epidermis is a thin cubical or pavement epithelium, 

 while the chitinous layer is of greater thickness and, since it is 

 secreted by the epidermis, is stratified parallel to the surface. Its 

 firmness is due to the chitin, which is unlike most organic substances 

 in its resistance to acids and alkalis; only under the action of 

 sulphuric acid and heat is it broken up into sugar and ammonia. 



