ON PHYMOSOMA VAETANS. 27 



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. 



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

 The multicellular papillae which secrete the hooks are shown. One of these 

 sense organs at the base of the hooks is also shown cut taugentially. 



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 papillse. 



PLATE IV. 



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 prseoral 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 circumcesophageal commissure 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 prseoral 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 prseoral 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 mesenteries 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 sheatb, 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 view of the head of Phoronis, seen from in 

 ront. 



Fig. 32. — A similar view of Phymosoma. 



