490 A, T. MASTERMAN. 
Lang was, I believe, the first to suggest an explanation of 
this difference as being accountable to the sedentary habits of 
Cephalodiscus, which has presumably resulted in a loss 
of all but one pair of the pharyngeal clefts and gonads. 
The same view has been held more recently by Willey,’ to — 
whom the assumption is apparently necessary for his theory 
of the primary origin of pharyngeal clefts, as a cleft, 
according to him, can apparently only arise between two 
contiguous gonads. 
According to Willey pharyngeal clefts have arisen pri- 
marily as branchial clefts, or clefts connected with respira- 
tion of the gonads, and they have only secondarily lost this 
function and become connected with the removal of water in 
relation to alimentation. 
In a paper read in 1898° I suggested a theory of the origin 
of pharyngeal clefts which attaches no primary importance 
to the contiguity of the clefts and the gonads in Entero- 
pneusta. It was assumed that, upon the first evolution of 
metameric representation in the trunk of Enteropneusta, 
the gonads and clefts became repeated together in regular 
order from a primary Pterobranchian arrangement of one 
hich is entirely opposed to, and 
incompatible with, Willey’s suggestions. ‘There appear to 
be several further difficulties in the way of accepting Willey’s 
hypothesis. Firstly, there is no evidence of any kind that 
the Pterobranchia or Phoronis ever had more than one 
pair of clefts or gonads, the probability of which has been 
urged by him. Secondly, there is doubt as to whether the 
clefts in Enteropneusta really subserve respiration of the 
gonads in a direct manner. ‘The necessity for such an 
pair 
arrangement does not appear obvious when we recall the 
existence of a large number of Annelid animals with large 
gonads embedded deep in the tissues, but with no special 
adaptation for direct aeration of these organs. Thirdly, the 
facts here described show that the larval Enteropneusta 
1 ¢Quart. Journ. Mier. Sci.,’ “ Enteropneusta,” Willey’s Zoological Results. 
2 «Report of the British Association ” (Bristol), 1898. 
