Chap. VI.] 



TRACHEA. 



215 



has directed attention to the deeper slits and larger 

 amount of haemoglobin in the Schizonemertini, as 

 being correlated with their habit of dwelling in mud 

 and other places in which the supply of oxygen is 

 small ; the ciliated cells of the groove clearly serve to 

 drive in currents of sea-water. 



In c and D (Fig. 92) it will be observed that the 

 lower part of the ganglionic mass is shaded more lightly 

 than the rest ; the cells that form this portion are 

 derived from the oesophagus, from the walls of 

 which, however, they subsequently become separated ; 

 and we observe, therefore, that an out-pushing from 

 the gullet goes to 

 meet the epiblastic 

 in-pushing from the 

 surface. 



An essentially 

 similar phenomenon 

 is to be observed 

 in the Eiitero- 

 piieusti (Balano- 

 glossus) and in the 

 Cliordata. (See 

 page 231.) 



In the traclie- 

 ate Artliropoda 

 we have examples of 

 in-pushings of the 

 surface adapted for 



the entrance not of water, but of air ; they are seen at 

 their simplest in Peripatus, where they are 

 distributed over the whole of the body. The tracheal 

 orifice leads into a pit, which traverses the dermis, 

 and widens out at its inner end ; from this the 

 tracheae arise as minute tubes, which rarely branch, 

 and only have a faint indication of the presence of the 

 spiral fibre, which, in higher Tracheates, gives so 



Fig. 93. Trachea? of Insect, showing the 

 Spiral Fibre. 



