G. A. DREW ON YOLDIA LIMATULA, SAY. 13 



plate, Fig. 19, mp. These were supposed by Mitsukuri 8 to be chiti- 

 nous, but they were later shown by Kellogg 1 "' to be muscles. They 

 start from the suspensory membrane as bundles of fibres, which 

 branch and spread out laterally as they descend. Many of the fibres 

 apparently find insertion in the wall of the plate itself, but not a few 

 are extended to the chitinous rods and are inserted over their surfaces. 

 Kellogg, 5 no doubt rightly referred movements of the plates, which 

 are common in the gills of mutilated specimens, and in the excised 

 gills themselves, to these muscles. The normal function of these 

 muscles will be made apparent later, in connection with the special 

 function of the gills. 



Each plate bears cilia along its ventral margin, and for a short 

 distance along each side, but not over its general surface. 



The cilia between the plates are arranged in rows that run 

 parallel with the margins of the plates, there being one or two rows 

 on each side of each plate. Near the ends of the chitinous rods, 

 the cilia on the edges of the plates disappear, and those between the 

 plates become modified, as described by Kellogg, 15 for holding the plates 

 together. They do not seem to be hooked, but are very numerous, 

 rather stout, and freely intermingle with the cluster on the adjoining 

 plate. The area covered by these cilia is rather limited, and above 

 them the plates are not ciliated. The cilia of the gills seem to drive 

 water between the plates and aid in lowering the gills. 



Each gill has two rather large bundles of muscles running its 

 entire length, one above, Fig. 19, ulm., and one below, llm. The lower 

 bundle is somewhat crescent shaped in cross-section, the concave 

 surface being directed upward and forming part of the ventral wall 

 of a large blood-space, bs., that is also continuous the length of the 

 gill. These longitudinal muscles were supposed by Mitsukuri, 8 whose 

 material was poorly preserved, to be fibrous tissue, but there seems to 

 be no doubt that they are muscles. Their contraction causes the gills 

 to shorten, a movement already described by Kellogg, 5 and a move- 

 ment necessarily of service in their special function. 



The cavities of the plates on the two sides of each gill open freely 

 into each other, and the cavities of all of the plates are put into 

 communication through the blood-space, bs., which is continued 

 upward between the plates as a narrow slit reaching to the upper 

 longitudinal muscle. Above this muscle, at the base of the suspensory 

 membrane, is another blood-space, also running the length of the 



