ATHROPODA 357 



separate ommatidia. This 'mosaic theory' has completely replaced the 

 view that each ommatidium formed a complete inverted picture. 



While the number of ocelli varies, the compound eyes are almost invariably 

 two in number, but can be constricted into four or eight. The single eye of 

 Daphnids is the result of fusion. There is always a large optic ganglion in the 

 optic nerve of each compound eye. As in vertebrates both ocelli and com- 

 pound eyes can shine in the dark. This is the result of the 'tapetum lucidum,' 

 formed in the Crustacea and arachnids of modified pigment cells, in the insects 

 by a rich tracheal network. 



The tactile organs, consisting of tactile hairs (fig. 80), are uniform 

 in structure. On the other hand the senses of hearing, taste, and smell 

 are subserved by varying organs. We know but little of these senses in 

 arthropods, although they are frequently well developed. The sense of 

 smell resides chieily in the antenna; and in the palpi of the jaws. The 

 organs are olfactory tufjules or cones (mocUlied hairs) which frecpiently 

 lie in pits in the skin. Similar organs in the mouth are probably con- 

 nected with taste. As organs of hearing ( ? ecpulibration) besides the 

 statocysts of the Podophthalmata and the tympanal organs of the Orthop- 

 tera, the widely distributed 'chordotonal' nerve ends of insects are to be 

 mentioned. 



The larger proportion of the alimentary canal is formed of ectodermal 

 stomodeum and proctodeum, wliile the entodermal portion (mesenteron) 

 usually forms but one-tliird of the total length. At ecdysis the chitinous 

 lining of the ectodermal parts, including the large chewing stomach, is 

 cast with the rest of the integument. The entire absence of ciliated 

 epithelium is noteworthy. Ciliated cells have never been found in 

 arthropods. 



The most constant portion' of the circulatory system is the heart, 

 which lies immediately beneath the back, enclosed in a more or less 

 distinct sac which, although called pericardium, is not a part of the 

 coelom. From the pericardium blood passes into the heart by openings 

 {ostia) right and left. Since the margins of the ostia project far into the 

 lumen of the heart and so form folds functioning as valves, the heart 

 itself may be divided into a series of chambers (fig. 67). The chambers 

 disappear when the heart is reduced to a sac. In small arthropods the 

 heart together with the whole vascular system may be lost. Since the 

 Annelida have a well-developed circulatory system, the loss in the arthro- 

 pods is secondary rather than primitive, and is explained by the fact that 

 with reduction in size the organization is simplified. The heart can thus 

 be lost in small Crustacea (many ostracods, copepods, and barnacles), 

 in the smaller arachnids (mites), and in insects (many aphids), while it is 

 present in allied forms. 



