THE ANCESTRY OF THE VERTEBRATES 515 



nephridium, which thus serves as ductus deferens or oviduct. 

 Typically, as in the diagram, a germ gland belongs with each 

 lateral coelomic cavity, but in actual cases they develop in only 

 a few y segments, and the associated nephridia become espe- 

 cially modified for the conveyance of the germ cells from the 

 body. 



If, now, the primitive vertebrate nephridia, germ-glands and 

 ccelom, as described in Chapter IX above, be compared with 

 the annelid condition, the similarities are found to be remark- 

 able. Here also the nephridia are at first strictly segmental, 

 although they no longer open to the outside independently, but 

 through the medium of a common Wolffian duct. Since, how- 

 ever, this develops in part from the ectoderm, it may have be- 

 gun as a simple external trough-like depression which ran 

 along the sides of the animal and connected the several indi- 

 vidual openings for the better disposal of their secretion. The 

 segmental subdivisions of the coelom are no longer continued 

 in the higher vertebrates but the mesodermic diverticula, which 

 appear clearly in Amphioxus, and in a more imperfect manner 

 in the others, suggest the former presence of transverse dissepi- 

 ments, and both dorsal and ventral mesenteries actually persist 

 as far back as the posterior boundary of the liver, beyond which 

 the ventral one disappears. Nor can it be said that the trans- 

 verse dissipiments are wholly lacking, since, although they no 

 longer divide the coelomic cavity, they are still represented in 

 the body wall by the intermuscular septa (myocommata) with 

 which the nephridia sustain in the embryo similar relationships 

 as in annelids (Fig. 139, d) . In both cases the germ glands 

 arise as localized portions of the ccelom (peritoneum), and the 

 presence of a single pair in the true vertebrates may be corre- 

 lated with the confluence of the several coelomic cavities into 

 a single one. The larger number of gonads in Amphioxus 

 indicates the former presence of a much larger number of 

 coelomic cavities. That in vertebrates as in annelids the 



Annelid. (c) Cross section of Selachian. (d) Longitudinal section of Selachian 

 in region of kidney. 



n, nerve cord; nc, notochord; g, " faserstrang " of Annelid; sh, sheath sur- 

 rounding nerve cord and notochord; d and r, muscle masses; en, intestine; nph, 

 nephridium; a and b, longitudinal blood vessels. 



