34 MORPHOLOGY OF 



&c., between the myotomes. The rods^ however, are seen in 

 adult specimens to have concentric markings in section, sug- 

 gesting that they are formed by a deposit around a central 

 core. In that part of the rod which lies just in front of the 

 point of divergence of the two horns there are two such cores. 

 The cores stain more deeply than the rest of the rods. 



InB. minutus the position and general appearance of the 

 notochord is similar to that in B. Kowalevskii, with the 

 exception that the lateral rods are not developed to the same 

 extent. The histology, however, is somewhat different. In 

 figs. 61 — 63 the appearance of the tissue is shown. Spengel 

 has stated that he is unable to find, in B. minutus, any 

 structure resembling the notochord of the Chordata, figuring 

 that organ as though composed of columnar cells. My own 

 observations give no support to this statement. No specimen 

 of notochord of any of the species which have been examined 

 by me present the appearance indicated by Spengel. On the 

 contrary, specimens preserved severally in picric acid, corro- 

 sive sublimate and acetic acid, Perenyi's fluid and osmic acid, 

 all equally show this body as made up of vacuolated tissue 

 strongly suggestive of the notochord of Chordata. This is 

 especially the case with regard to that of B. minutus. Fig. 

 61 is taken from a section of B. minutus in front of the lu- 

 men. The sheath is here very slightly developed. It will be 

 observed that the nuclei are fewer in number than in B. Ko wa- 

 le v s k i i, and that they are gathered round the upper centre. The 

 section shown in fig. 63 is from the same in the region of the 

 proboscis stalk. It shows the diamond-shaped section of the 

 skeletal rod and the concentric markings upon it. The noto- 

 chord itself is here elliptical in section, and the strands of 

 protoplasm running across it are well seen. 



Fig. 63 shows its appearance in front of the point at which 

 it opens. Dorsally its wall is very thick and solid, while 

 ventrally it is comparatively thin. Behind this part the 

 skeletal rod divides into two divaricating horns as in B. 

 Kowalevskii. 



In the other species the structure of the notochord is 



