376 
E. RAY LANKESTER. 
chamber is divided, for a part of its extent, into a number of 
nearly horizontal passages which may be compared to the 
series of parallel adherent tube-like passages connecting the 
gill pouches of Myxine with the branchial pore of that animal. 
When the development of Myxine can be studied, I should be 
anxious to inquire whether the tube-like passages in question 
are formed by the septation of a primitively simple subopercular 
cavity through the outgrowth of interbranchial septa as in 
Amphioxus. 
The number of gill-slits, counting each of the primary slits as 
two, observed by Johann Muller in a small transparent speci- 
men, was 50, in individuals of an inch long 80 to 100. In indi- 
viduals a little over an inch in length I have counted 96 slits, 
and in larger specimens (nearly two inches long), 124. To arrive 
at the number of primary slits we have to halve these figures, 
since each pair of slits is formed in the way above noted. 
The independence of the gill-slits in relation to the meta- 
merism of the body wall is related to the following facts. 
(1) The myotomes increase in volume during growth but not 
in number. (2) The whole pharyngeal region of the body 
increases in volume during growth, and the point at which the 
perforations cease, though it remains throughout life (after a 
size of three quarters of an inch has been reached) in 
approximately the same relative position to the superjacent 
myotomes, viz. coincident with a vertical line drawn through 
the anterior angle of the myotomes 27 to 29, yet advances 
gradually backwards from the former to the latter as growth 
goes on. (3) The pharyngeal slits do not increase in width, 
and the increase of the pharynx is made by new local growth at 
its posterior end. Accordingly new slits are formed in the new- 
growing region of the pharynx. It thus results that organs which 
are fixed in position in relation to a particular myotome — for 
instance, the “ atrio-coelomic funnels,” to be described below, of 
which I have spoken in my earlier paper as “ the brown canals,” 
are found to vary in their relative position to the perforated 
region of the pharynx. In small specimens the atrio-coelomic 
funnels are in the same plane with the non-perforated termina- 
