ON PHYMOSOMA VARIANS. 99 



dark layer of muscle-fibres and internally the secreting epithelium. The 

 breaking up of the lumen into numerous side chambers is also shown in 

 this figure. 



Fig. 20. — A portion of the same under a high power. The secreting 

 epithelium is seen crowded with granules ; at their free edges these cells 

 form vesicles, which break off and fall into the lumen. 



Fig. 21. — A section through parts of several of the hooks on the 

 introvert. The multicellular papilla3 which secrete the hooks are shown. 

 One of these sense organs at the base of the hooks is also shown cut 

 tangentially. 



Fig. 22. — A view of the base of the two ventral retractor muscles, 

 showing the generative organ. The ventral nerve-cord lies between the 

 muscles and dorsal to the generative ridge. The circular and longitudinal 

 muscles are also shown, and the outline of the papillae. 



PLATE X. 



Fig. 23. — A section through the antero-dorsal corner of the brain, to 

 show the blind end of the sense-pit. The cells lining the inner end of the 

 pit are crowded with pigment. A few cells of the ectoderm of the preeoral 

 lobe are seen, and part of the blood sinus in which the brain lies. 



Fig. 24. — An oblique section through the lateral part of the brain, 

 showing the origin of the circumoesophageal commissvire and of the 

 lophophoral nerve. This figure and the three succeeding ones show 

 the arrangement of the ganglion-cells, the giant cells, and nerve-fibres. 



Fig. 25. — A section through the brain, transverse to its long axis, and 

 nearer to the middle line than the preceding figure. It shows the fusion 

 of the brain with the ectoderm of the praeora,! lobe, and the commencement 

 of the prseoral lobe nerve. 



Fig. 26. — A section in a place parallel to the preceding, but still 

 nearer to the median line, it shows the origin of the preeoral lobe nerve. 



Fig. 27. — A horizontal section through the posterior part of the brain 

 at right angles to the preceding. This shows the histology of the giant- 

 cells and their relative size. 



Fig. 28. — A longitudinal median section of the ventral nerve-cord, 

 showing the arrangement of the ganglion-cells and fibres, and .the mesen- 

 teries which attach the cord to the ventral body wall. 



Fig. 29. — A transverse section of the nerve-cord, showing a mesentery 

 from the ventral body wall, the arrangement of ganglion-cells and nerve- 

 fibres, the connective-tissue sheath, and the lateral muscles which run 

 along each side of the nerve-cord. 



Fig. 30. — An ovum and some of the coelomic corpuscles. The ovum 

 shows the granular protoplasm, the large nucleus, and the zona radiata. 



Fig. 31. — A diagrammatic vie* of the head of Phoronis, seen from in 

 front. 



Fig. 32.— a similar view of Phymosoma. 



