ON THE DIPLOCHORDA. A491 
have a pharynx in which there are simple paired pleuro- 
chords, terminating in lateral oral grooves. 
Morgan! has shown that this portion of the pharynx of 
Tornaria, here-identified as the pleurochords, grow out- 
wards in the course of development into serial pouches, which 
eventually reach the ectoderm and form the pharyngeal clefts, 
the pleurochords themselves remaining as the two halves of the 
dorsal part of the adult pharynx, into which the clefts open in 
the adult, whilst the ventral part expands to form the ventral 
portion of the adult pharynx. As soon as the first pair of 
pharyngeal clefts arises in Tornaria its pharynx is then, in 
its essential features, comparable with that of the Ptero- 
branchia, passing on to that of the adult Enteropneustan 
by the production of further pharyngeal clefts along the 
course of the pleurochords. The conclusion appears to be 
fairly evident that the Enteropneustan pharynx must be 
regarded as a further elaboration of the type found in 
Pterobranchia, or, in other words, that the Entero- 
pheusta are essentially Diplochorda, though this is 
disguised in the adult by a general vacuolation of most of 
the pharynx and the production of a large number of pharyn- 
geal clefts. Harmer? has, on more than one occasion, 
recorded an emphatic objection to the adoption of this term, 
but such results as the above seem to show that it expresses 
a fundamental character of these animals. LHven if the name 
be not adopted in classification, the underlying character of 
the presence of true lateral pharyngeal pleurochords can- 
not be ignored. Harmer goes no further than comparing 
these organs in Pterobranchia with “the dorsal part of 
_the pharynx of Balanoglossus,’ a comparison which 
agrees precisely, so far as it goes, with the view here 
expressed. 
Recent work upon Rhabdopleura?’ (Schepotieff) has 
shown that this type also possesses two pharyngeal pleuro- 
1 *Journal of Morphology,’ vol. ix. 
Ze LOOlPANZEICeL.: KX, 89 ene. elit: 
> Bergens Museums, Aarbog. 1904; ‘Zvolog. Anz.,’ xxviil, 1905 
