THE ENTBROPNEUSTA. 25 



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 simultaneously 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. Kowalevskii it is on the left side throughout 

 life. In all species it perforates the nerve-ring of the stalk. 



