An endeavour to show that the tracheae arose from setiparous sacs. 513 



comes interesting if the connection between hair sacs and tracheœ can 

 be established. 



Before going further, it may perhaps be well to point out that, 

 beside the ordinary setae of the Annelids, certain bristles were modi- 

 fied into supporting organs. Each parapodium or branch of a para- 

 podium developed one large bristle which hardly appeared above the 

 surface of the body and whose functions were almost purely skeletal, 

 giving firmness to the whole parapodium. These supporting bristles 

 are known as acicula. We shall find that while some tracheae are 

 to be referred to ordinary bristle sacs, others can only be referred to 

 acicular sacs. 



Let us take the different groups of the Tracheata separately, the 

 order chosen being that which best suits the exigencies of our ar- 

 gument. 



The Hexapoda. 



As, in discussing origins, we must necessarily avoid specialised 

 forms, we naturally confine our attention to the primitive conditions 

 still visible in the embryos. 



Fig. 2 is the embryo of Melolontha^ and Fig. 3 the same of Hy- 

 drophilus. A row of stigmata is seen on each side just above the 

 bases of the limbs. They occur from the first thoracic to almost the 

 last abdominal segment. When we compare the position of these 

 openings and the probable position of the acicular glands of the ori- 

 ginal Tracheatan-Annelid , their origin from such glands seems very 

 probable. 



In order to make this clearer it is necessary to make some 

 suggestion as to the origin of the appendages of the Tracheata. 



In the Crustacean-Annelid, which is the term I have proposed i) 

 for the Annelidan ancestor of the Crustacea, both branches of 

 the parapodia were originally retained, the ventral as gnathobases 

 or jaw-pieces, and the dorsal as swimming plates. These latter 

 were then gradually transformed into ambulatory legs. In the Tra- 

 cheatan-Annelid which , according to the theory , early migrated 

 to the land, the ventral parapodia would be those most probably 

 developed into locomotory limbs. There could be no question of using 

 the ventral parapodia of the trunk limbs as accessory jaws, because 

 the head was not bent round in these early land Arthropods, as I have 



1) The Apodidse, a morphological study. Macmillan 1892, 



