458 TRACHEATA. 



same for all the Tracheata so far investigated, and is also very 

 much the same as for the Chaetopoda. There is a division into 

 somites; each containing a section of the body cavity. In the 

 cephalic section of the mesoblastic bands a section of the body 

 cavity is also formed. In Arachnida, Myriapoda, and probably 

 also Insecta, the body cavity is primitively prolonged into the 

 limbs. 



In Spiders at any rate, and very probably in the other groups 

 of the Tracheata, a large part of the mesoblast is not derived 

 from the mesoblastic plate, but is secondarily added from the 

 yolk-cells. 



In all Tracheata the yolk-cells give rise to the mesenteron 

 which, in opposition, as will hereafter appear, to the mesenteron 

 of the Crustacea, forms the main section of the permanent 

 alimentary tract. 



One of the points which is still most obscure in connection 

 with the embryology of the Tracheata is the origin of the 

 embryonic membranes. Amongst Insects, with the exception 

 of the Thysanura, such membranes are well developed. In the 

 other groups definite membranes like those of Insects are never 

 found, but in the Scorpion a cellular envelope appears to be 

 formed round the embryo from the cells of the blastoderm, and 

 more or less similar structures have been described in some 

 Myriapods (vide p. 390). These structures no doubt further 

 require investigation, but may provisionally be regarded as 

 homologous with the amnion and serous membrane of Insects. 

 In the present state of our knowledge it does not seem easy to 

 give any explanation of the origin of these membranes, but they 

 may be in some way derived from an early ecdysis. 



