62 MORPHOLOGY OF 



V. g. Nervous concentration in the line of the gill-slits. V. msc. Ventral 

 muscles. Flv. Valve of gill-slit. V. n. s. Ventral nervous cord. 



Fig. 64. — Diagrammatic longitudinal vertical section of B. minutus, to 

 show the arrangement of the nervous system. [The openings of the 

 gill-slits are indicated, though of course not visible in a section of this kind.] 



Fig. 65. — Diagram of nervous system of B. Kowalevskii as seen 

 from the dorsal surface. The ventral cord and the ring round the pharynx 

 are indicated in broken lines. The sheath of nervous tissue covering the 

 proboscis is indicated by shading, as though the tissues were transparent. 

 The gill-slits are shown on one side only. 



Figs. 66 — 73 illustrate the structure of the skin and nervous tissues of B . 

 minutus. 



Fig. 66. Nearly median longitudinal vertical section of the middle third 

 of the central nervous system, showing origin of two of the cords 

 connecting central nervous system with the skin. Their union with 

 the skin is not here shown, (v. Fig. 68.) Obj. A, long tube, oc. 2. 

 Fig. 67. Longitudinal vertical section through the side of the central 

 nervous system, showing the relation of the neural and proboscis pores 

 to each other, &c. The wall of the heart is cut in this section. 

 As the section is taken through the side of the central nervous system 

 its continuation into the dorsal nerve-cord is not visible. Obj. A, 

 oc. 2. 

 Fig. 68. Transverse section of the central nervous system at end of 



neural tube. Obj. A, long tube, oc. 2. 

 Fig. 69. Transverse section of the central nervous system behind the 

 neural tube, showing attachment of dorsal cord to the skin. Obj. A, 

 oc. 2. 

 Fig. 70. Longitudinal section of the anterior end of the ventral nerve- 

 cord. Obj. D, oc. 2. 

 Fig. 71. Transverse section of ventral nerve-cord. Obj. A, oc. 2. 

 Fig. 72. Longitudinal section of skin in lateral region. Obj. A, oc. 2. 

 Fig. 72 A. Transverse section of skin in the space between the gill-slits. 



Obj. D, oc. 2. 

 Fig. 73. Longitudinal horizontal section through the back of the collar, 

 showing the relations of the peripharyngeal nerve-ring. 



Fig. 74 («, i, c). — Three sections taken through the anterior, middle, and 

 posterior thirds respectively of the central nervous system of B. salmoneus. 



Fig. 75. — Section of a wrinkle of the skin of the middle third of the pro- 

 boscis of B. salmoneus. Obj. D, oc. 2. 



Fig. 76. — Teased out osmic acid preparation of the skin of the collar of 

 B. Robinii. The cells remain attached to each other by their heads. The 



