7 6 



ANATOMY OF AMPHIOXUS. 



a young larva with some six or seven gill-slits in the form 

 of two longitudinal thickenings of the integument on the 

 ventral surface of the body. These are at first solid, but 

 eventually become hollowed out so as to enclose a longitu- 

 dinal canal on each side. This is the so-called metapleural 

 canal or lymph-space. The thickenings enlarge to the 

 extent of forming two well-marked folds of the body-wall ; 

 namely, the metapleural folds. 



The next stage is marked by the formation of two small 

 solid longitudinal ridges on the inner opposed faces of the 

 metapleural folds (Fig. 36). It is by the subsequent 



Figs. 36 and 37. Schematic transverse sections through post-pharyngeal 

 region, illustrating mode of origin of atrial chamber. (After LANKESTER and 



WlLLEY.) 



ao. Aorta, b.c. Coalom. r.m and l.m. Right and left metapieural folds, s.a.r. Sub- 

 atrial ridges, which fuse together to form the floor of at, the atrium, int. Aliment- 

 ary canal, s.i.v. Sub-intestinal vein. 



meeting and coalescence of these subatrial ridges that the 

 atrial cavity becomes enclosed as a small median tube lined 

 by ectoderm. 



As soon as it has become closed off from the exterior, 

 the atrial tube commences to grow in size, and it gradually 



