430 ZOOLOGY 
part of the trunk which succeeds the branchial region. The 
ova do not escape singly through the external orifice, but the 
whole follicle breaks away and then disintegrates. 
The systematic position of Balanoglossus has given rise 
to much divergence of opinion amongst zoologists. Bateson’s 
Fie. 247.—Section through Balanoglossus behind the region of the gill-slits. 
2. Alimentary canal supported byadorsal 7. Nerve plexus at the base of the 
and ventral mesentery and showing epidermis. 
the dorsal and ventral grooves lined 8&8. Longitudinal muscle fibres. 
by long cilia. 9. Connective tissue cells filling up the 
3. Dorsal nerve cord in the skin. coelom. 
4. Dorsal blood-vessel. 10. Ventral blood-vessel. 
5. Ovarian follicles, opening to the 11. Ventral nerve cord in the skin. 
exterior. 
recent researches on the embryology of this form, however, 
justify us in placing it amongst the Chordata, but it has 
affinities in at least two other directions. Certain species, 
in the course of their developement, pass through a larval 
stage termed the Yornaria, which shows the closest resem- 
blance to the characteristic Bipinnaria larva of the Echino- 
dermata. The presence of this larva in the ontogeny of 
these two groups seems to point to some connection between 
their remote ancestry, and this to some extent is emphasised 
by the fact that the Echinodermata, like Balanoglossus, are 
ciliated externally. Again, the structure of the body-wall, its 
external ciliation, the form of the body, the absence of seg- 
mentation in the muscles, the structure of the generative 
organs, and perhaps the proboscis, are all features which recall 
the similar parts in the Nemertines. 
