352 BALANOGLOSSUS. 



DIAGRAM XVIII. 



Balanoglossus. 



The figure Bal. shows the form of B. kowalevskii, pr. the proboscis, ' 

 col. the collar, g. the region with gill-slits. (After Bateson.) 



The figure Sec. is a longitudinal section of the anterior region, showing 

 P. p. the proboscis pore, mo. the mouth, Nch. the notochord, ht. the heart, 

 gl. i. the glandular sac, c. n. s. the central nervous system, D. n. s. the 

 dorsal continuation of it, v. n, s. the ventral nerve, g. the gill-slits, and 

 the gut. (After Bateson. ) 



Figure 1 1 shows a cross section in the region of the gill-slits, showing 

 /. the pore of a gill-slit {g. si.), d. n. the dorsal nerve, d. b. the dorsal 

 blood-vessel, r. the reproductive organs, b. c. the body-cavity, v. n. the 

 ventral nerve, and above that the ventral or nutritive region of the gut. 

 (From Korschelt and Heider, after Spengel.) 



Lar. represents a, larva of B. kowalevskii, p. the proboscis, t. the 

 collar, g. the openings of two gill-slits. (After Bateson. ) 



Tor. is a Tornaria larva, showing m. the mouth, an. the anus, and 

 the ciliated band and ring as dark lines. (From Balfour, after Metschni- 

 koff.) 



Figs. l-io (after Bateson) illustrate the development : — i. a section of 

 the blastosphere ; 2. the invagination which forms the gastrula, end. the 

 endoderm ; 3 and 4. sections of a gasttula, Ac. the archenteron, j. c. the 

 segmentation ■ cavity ; 5-7 show the origin of the coelome pouches — 

 m', m", ml", forming fc', bc^, be", the interrupted dark line is the meso- 

 derm ; 8 is a cross section of the embryo, showing g. the gut, c. n. s. 

 the central nervous system, be', the lateral parts of the body-cavity ; 

 in 9 and 10 the mesoderm is a continuous dark line, be' the body-cavity. 

 Nek. the notochord, c. n. s. the central nervous system, D. b. v. the 

 dorsal blood-vessel. 



