128 ESSENTIALS OF ZOOLOGY 



of sleeping sickness to man, and it is also concerned in certain 

 animal diseases x (see page 28). 



Insects are to be distinguished from the eight-legged 

 spiders, mites and ticks which belong to the Arachnida. Ticks 

 may also serve to communicate disease, as Texas Cattle Fever 

 in America and Louping-ill to sheep in Britain. 



The study of insects has therefore become an important 

 branch of Zoology relating to agriculture and to medicine. 



General Considerations. A few remarks may now be made 

 as to the main features of the preceding metazoon groups. 

 The Coelenterates are diblastic, and the two layers, the ecto- 

 derm and the endoderm, are related, as in the gastrula stage 

 of development of the other groups. The Platyhelmia are 

 provided with a mesoderm interposed between the two primary 

 layers, and it is resolved into a gonocoel and mesenchyme and 

 muscles. The mesoderm is invaded by ectodermal invagina- 

 tions which branch in the spaces among the mesenchyme 

 cells and end in flame cells or solenocytes. These primitive 

 kidneys are called protonephridia. The Eotifera are very 

 similar in constitution, but the space is large and distended 

 by a fluid, and the gonocoel projects into it from a pouch at 

 the posterior end of the alimentary canal. The mesenchyme 

 spaces have been converted into a primary body cavity or 

 schizocoel. The gonocoel is endodermal in origin. The 

 schizocoel possesses protonephridia of a nature and develop- 

 ment quite similar to those of the Platyhelmia. With the 

 exception of the gonocoel, the structure of the rotifer is that 

 of the embryo or the larva of the other Trochozoa. The 

 remaining Trochozoa have the mesoderm of the post-embryo 

 or post-larva and adult resolved into two distinct spaces : 

 (1) the primary body cavity, homologous with the primary 

 spaces into which the blood penetrates ; (2) the secondary 

 body cavity or coelom, homologous with the gonocoel, for it 

 has a similar origin and primarily the same function. 



With relation to the primary body cavity, protonephridia 

 are developed from the ectoderm and function throughout 

 life not only in the Eotifers but in lowly groups allied to the 



1 1913, Castellan! and Chalmers, Manual of Tropical Medicine; 1911, 

 James and Liston, A Monograph of the Anopheline Mosquitos of India, 



