600 



INVERTEBEATA 



CHAP. 



During the larval life the excretory organs are formed. These 

 are true nephridia comparable to those of Oligochaeta, Nemertinea, 

 and Platyhelminthes. They consist of tubes ending internally in 

 tufts of solenocytes or flame cells. Their origin has been described 

 by Goodrich (1909). When they are first discernible they consist of 

 small pear-shaped masses of cells, almost certainly ectodermal in 

 origin, situated at the lower borders of the primitive gill-slits. They 

 rapidly become hollow and long solenocytes grow out from their 

 inner ends. These solenocytes extend up the left side of the body 



s.iv 



Fig. 438. — Diagrammatic transverse sections through the hinder region of two young 

 specimens of Amphioxus lanceolatiis, in order to show the origin of the sclerotomes. 

 (After Hatschelt, slightly altered.) 



A, through a larva with five primary gill-slits— the lower sclerotome is just beginning to be formed. 

 B, through a specimen just after the metamorphosis — the lower sclerotome has extended upwards be- 

 tween the myotome and the notochord, and the upper sclerotome has given rise to the ^n-ray cavity. 

 ch, notochord : d.a, dor.sal aorta ; myx, myocoele ; n.c, nerve cord ; sd^, upper sclerotome ; scl^, lower 

 sclerotome ; mi\ septum dividing myocoele from splanchnocoele ; s.i.v, sub-intestinal vein ; spl, splanch- 

 nocoele ; t'./.r.c, ventral fin-ray cavity. 



to the level of the dorsal aorta (so/. Fig. 439). In the adult the 

 solenocytes are much shorter than in the larva. One very large 

 nephridium hes in the forward extension of the right collar -cavity, 

 and opens behind into the mouth. This is called Hatschek's 

 nephridium. 



As the larva grows older it seeks lower and lower levels in .the water. 

 Finally, after about three months of larval life, the metamorphosis 

 sets in, and during this period the larva frequently lies on its side on 

 the bottom. Before metamorphosis begins, however, the rudiments 

 of the genital organs make their appearance. Their development 

 has been elucidated by Boveri (1892). They arise as small buds of 

 cells {germ, Fig. 440), which sprout from the anterior lower angles of 



