DEVELOPMENT OF BALANOGLOSSUS KOWALEBVSKIL. 105 
greatly in size, and assumes a vacuolated appearance closely 
resembling that of the notochord of young Lampreys and 
Elasmobranchs. 
The skeletal rods attain a considerable size. Their anterior 
ends unite, forming a single bar, while their posterior ends 
diverge, partially enclosing the gut. This whole structure 
forms the support of the proboscis. 
From its development, position, relations to surrounding 
parts, histology and function it appears to me to be com- 
parable with the notochord of the Chordata, and this name is 
strictly appropriate to it. Even if the suggestions which will 
be made hereafter as to its phylogenetic significance be not 
accepted, this rejection would in no way militate against the 
fact that this structure is to all intents and purposes a noto- 
chord, which can only be designated as a longitudinal dorsal 
supporting rod, derived from the hypoblast. 
The nervous system afterwards attains a great develop- 
ment (fig.60). The dorsal cord in the collar sinks further and 
further from the skin, being (in B. Kowalevskii) connected 
to it by a mesentery. The lumen is in this form less developed 
than in B. minutus, &c. The ventral cord is the next to 
appear, and almost simuitaneously with it arises the deposit of 
nervous tissue in the skin at the base of the proboscis. This 
deposit afterwards attains a great extent, forming a thick band 
round the proboscis stalk. It may be noticed that this nerve- 
ring has practically the same relation to the proboscis that the 
ring of ganglia in Nemertines presents, the proboscis of Balano- 
glossus being, however, permanently protruded, and the nerve- 
ring still in the skin. Both these nerve-rings agree in being 
traversed in Nemertines by two and in Enteropneusta by one 
pore communicating from the exterior to sacs which were 
originally archenteric diverticula. 
Body Cavities.—As before mentioned, the left horn of the 
anterior body cavity comes to open by the proboscis pore to 
the exterior. This opening is median and dorsal in other 
species, but in B. minutus it is on the left side throughout 
life. In all species it perforates the nerve-ring of the stalk. 
