332 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



EXPLANATION OF PLATE XVIII. 



All the figures except Nos. 1, 2, ia, and 13 were drawn with the aid of the 



camera. 



Fig. 1. A large Puget Sound individual from the left. Natural size. 



Fig. 2. A regularly shaped California individual from the left; x2. 



Fig. 2a. Same individual; dorsal view. 



Fig. 3. The disk, showing the arrangement of the plates, and the lines 

 indicating the boundary of their central thickened portion; 

 br, branchial siphon; at, atrial siphon; c, central plates; /, 

 lateral plates; p, peripheral folds; * points where the ac- 

 cessory central plates may be formed. Natural size. 



Fig. 4. Cross-section through a plate and the two hinges (h, h' ) at its 

 edges, showing the yellow layer (y. I), test papilla; (t. p), 

 and ectoderm (ec); X28. 



Fig. 5. The yellow layer, more highly magnified, showing an isolated 

 mass of yellow substance surrounding a bladder cell (c) 

 and two masses (a, b) which have just fused with the yellow 

 layer; x 830. 



Fig. 6. Section through the ectoderm and part of the cellulose layer, 

 showing the remains of ectoderm cells imbedded in the 

 latter; x 830. 



Fig. 7. A group of bladder cells. 



Fig. 8. Bladder cells, overstained in Iron - Hematoxylin to show their 

 slender processes into the test; x 830. 



Fig. 9. Various stages in the formation of the yellow substance about the 

 bladder cells; x 830. 



Fig. 10. A test papilla showing the attachment of a muscle fibre (m) by 

 means of the ectodermic pockets (pt); a and d are the points 

 where the ectoderm has been torn from the test; x 380. 



Fig. 11. The muscles of the disk from within; m. s, the short exterior 

 muscle fibres; pc. b, the peripharyngeal band. The outlines 

 of the plates are indicated by the heavy black lines; x 2. 



Fig. 12. Branchial sac of a very young individual; o, outer vessels; *, 

 intermediate vessels; 11, points of union between the outer 

 and intermediate vessels; x 130. 



Fig. 13. An individual with the ventral part of the test removed, from the 

 right side, showing the course of the digestive tract. * in- 

 dicates the position of the heart; x 2. 



Fig. 14. A younger individual; ventral view, but slightly from the right, 

 showing the ovary (o) and testis (t). Test entirely removed; 

 X5' 



