226 MAETIN F. WOODWARD. 



tlie gill, NucLila approaclies the Diotocardia in so many 

 other respects that one would naturally have expected a very 

 close agreement on this point. In the face of the unexpected 

 diiference one feels some doubt as to the full value of 

 generally accepted views on the relations of these forms. 



The Branchial Granglia. — On the outer side of each 

 gill and close to its anterior point of attachment is situated 

 a very conspicuous hemispherical swelling (figs. 5, 6, and 7, 

 hi. g.). These protuberances, which were described by Dall 

 as blood-sinuses, are caused by the presence of a large 

 ganglion, situated on the branchial nerve. The branchial 

 ganglia are the most conspicuous gauglionic swellings on 

 the nervous system. In section (fig. 19) they exhibit a great 

 accumulation of nerve-cells, arranged in two layers round 

 the periphery of the ganglion, a narrower outer and a broader 

 inner layer, the two being separated by a very narrow band 

 of fibrous tissue. The centre of the ganglion is occupied by 

 a great mass of fibrous tissue, the bundles of which run in 

 various directions. Near the periphery of the central mass 

 are some curious dim bodies, which at first sight suggest 

 large ganglionic cells ; but the entire absence of nuclei and 

 the want of sharpness of outline lead me to conclude that 

 they are in reality bundles of fibre, rather more closely 

 packed than usual. 



A very large nerve is given off from the ganglion to the 

 gill, and from this is derived that very conspicuous layer of 

 nerves (tig. 15, n, I.) following the course of the efferent 

 branchial vessel. 



The Osphradium. — Dall has figured a small hemispheri- 

 cal structure, situated somewhat nearer the middle line than 

 is the branchial ganglion (blood-sinus of Dall), which he 

 thinks may represent the osphradium. His description of 

 the position of this organ is, however, not very clear; and a 

 comparison of his figure (op. cit., pi. xxx, fig. 2) with my 

 fig. 6 suggests that the gill he represents is the right gill 

 seen from below, in which case his osphradium would in 

 reality be situated externally to the branchial ganglion, its 



