THE ENTEROPNEUSTA. 33 



(figs. 57 and 56). Immediately behind this downward exten- 

 sion lies the anterior end of the united skeletal rods, which 

 here attains its greatest thickness, almost filling the sheath of 

 the notochord, the tissue of which is here almost suppressed. 



In old specimens the shape of the anterior parts of the 

 notochord becomes rather irregular in section. 



In that part of its course which lies behind the proboscis the 

 notochord in the adult is more or less elliptical in section, 

 containing a large and somewhat irregular lumen. Its tissue 

 is here greatly reduced, and this reduction appears to progress 

 regularly as the animal grows older. In fig. 60 the appearance 

 of the notochord in such an old adult is shown. Degeneration 

 has progressed far, leaving the notochord as a space surrounded 

 by vacuolated cells enclosed in a sheath. With this sheath are 

 connected the skeletal rods, which attain a great size. Cen- 

 trally, on the dorsal side, between the notochord and the gut, 

 lies the principal rod ; this is formed by the uniting of the two 

 rods (figs. 37, 38, &c.), whose development has already been 

 described. This fused portion is now diamond-shaped in sec- 

 tion ; its lower angle causes a dorsal ridge to project into the 

 mouth cavity. Laterally are placed two long rods, which 

 are continued into the central rod and notochordal sheath 

 anteriorly. To these lateral rods are attached large bunches 

 of longitudinal muscles, by which, doubtless, the notochord 

 may be pulled backwards, and the proboscis retracted so as to 

 shut the mouth (fig. 60). 



Posteriorly the median rod divides into two, and the opening 

 from the notochordal lumen into the gut lies in the angle 

 formed by the separation of these two diverging rods 

 (fig. 57). 



A considerable deposit of " structureless" substance takes 

 place, filling up the spaces in the proboscis stalk, and forming 

 a partial sheath around the perihsemal cavity. "Whether this 

 substance is chitinous or of some other material I am unable 

 to say. The ensheathing parts of it have exactly the histolo- 

 gical appearance presented by the ''structureless" substance, 

 which in Amphioxus is continued from the notochordal sheath, 



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