58 MORGAN. [Vou. IX. 
A pair of reproductive gonads is present at this stage 
behind the last gill-slit, but a description of these will be 
deferred till a later stage. 
We may now pass to a description of the oldest worms of 
the series that had been kept from June 24 to July 17, drawn 
in Fig. 11, Pl. II. It is unnecessary to describe completely the 
structure of the specimens, as a great deal of repetition would 
be necessary. The points calling for special attention are as 
follows : 
Seven pairs of gill-slits open at the surface behind the 
collar; the anterior ones are very large compared with the 
preceding stages. The external openings are not so extensive 
as the gill-pouches themselves ; these are prolonged both above 
and below the openings, as explained in the preceding section. 
The rods are more largely developed than before. A cross- 
section of a tongue-bar and its posterior between-bar is given 
in Fig. 76, taken from a horizontal series. The rods are shaded 
darker, and it will be seen that the rods in the tongue-bar are 
widely separated, while those in the between-bar are fused 
together except at the innermost end, where they show a 
double origin. 
The body-cavity in the tongue-bars fills nearly the whole 
of the interior of the bar. A small cavity is seen on the 
inner side of the bar surrounded at the sides by the endo- 
derm, and on the outer side by the wall of the body-cavity. 
I believe this to represent a blood-vessel, although I have no 
definite proof of it. In the between-bar the body-cavity is con- 
fined at this level (where it opens to the exterior) to the outer 
region of the bar, where the bar is covered by ectoderm. 
A small triangular space is found towards the inner side 
of the body-cavity, bounded by endoderm and mesoderm. 
The space contains a coagulated material resembling blood, 
and the space seems to be a blood-vessel. At times, a space 
is found between the knobbed ends of the chitin-rods, but I 
cannot say that this is a blood-vessel. Both tongue-bars 
and between-bars have long cilia over the adjacent sides of 
the bars. 
