430 ZOOLOGY 



part of the trunk which succeeds the branchial region. The 

 ova do not escape singly through the external orifice, hut 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 



Fig. 247. — Section through Balanoglossus behind the region of the gill-slits. 



2. Alimentary canal supported by a dorsal 7. Nerve plexus at the base of the 



and ventral mesentery and showing epidermis. 



the dorsal and ventral grooves lined 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 TorTMria, which shows the closest resem- 

 blance to the characteristic BipinTiaria 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 cHiation, 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. 



