2 12 THE ORIGIN OF VERTEBRATES 



original opercular ducts by which the generative products were con- 

 veyed to the uterine chamber, i.e. to the chamber of the thyroid 

 gland, and thence to the common genital and respiratory cavity, and 

 so to the exterior. 



It is easy to picture the sequence of events. First, the generative 

 glands, chiefly confined to the cephalic region, communicating with 

 the exterior by separate ducts on the inner surface of the operculum 

 as in Limulus. Then, in connection with the viviparous habit, these 

 two oviducts fused together to form a single chamber, covered by the 

 operculum, which opened out to the exterior by a single opening as 

 in Scorpio : or, in forms such as Eurypterus, in which the operculum 

 had amalgamated with the first branchial appendage and possessed a 

 long, tongue-like ventral projection, the amalgamated ducts formed 

 a long uterine chamber which opened internally into the genital 

 chamber — a chamber which, as in Thelyphonus, was common with 

 that of the two gill-chambers, while at the same time the genital ducts 

 from the cephalic generative material opened into two uterine horns 

 which arose from the anterior part of the uterus, as in Thelyphonus. 



Such an arrangement would lead directly to the condition found 

 in Ammoccetes, if the generative material around the brain lost its 

 function, owing to a new exit for generative products being formed 

 in the posterior part of the body. The connection of the genital duct 

 with this cephalic gland being then closed and cut off by the brain- 

 case, the position of the oviducts would still be shown by the ciliated 

 grooves opening into the folded-down thyroid tube, i.e. the folded- 

 down horns of the uterus ; the uterus itself would remain as the 

 main body of the thyroid and still open by a conspicuous orifice into 

 the common respiratory chamber. Next, in the degeneration process, 

 we may suppose that not only the oviducts opened out to form the 

 ciliated groove, but that the uterine chamber itself also opened out, 

 and thus formed the endostyle of Amphioxus and of the Tunicata. 



It might seem at first sight improbable that a closed tube should 

 become an open groove, although the reverse phenomenon is common 

 enough ; the difficulty, however, is clearly not considered great, for it 

 is precisely what Dohrn imagines to have taken place in the conversion 

 of the thyroid of Ammoco^tes into the endostyle of Amphioxus and 

 the Tunicata ; it is only carrying on the same idea a stage further to 

 see in the open, ciliated groove of Ammocectes the remains of the 

 closed genital duct of Limulus and its allies. 



