462 
FLORENCE BUCHANAN. 
have feathery filamentous gills known as t richobranchs. 
The development of such a gill from a Schizopod gill is seen 
in the individual development of Stenopus, a form closely 
allied to Penaeus. Here a gill consisting of a shaft with 
two opposite rows of rays is first formed, resembling the 
gill of one of the Euphausidae (fig. 3). These rays, instead of 
becoming lobed, become longer and narrower, and other new 
rows of lobes appear on the shaft, which in their turn increase 
in length and decrease in width. These secondary rays do not 
spring so regularly from the shaft as the primary ones. Thus 
a typical trichobranch is formed. 
The other kind of gill, that of the Crab and most short-tailed 
Decapods, as well as of some of the long-tailed forms (e.g. 
Palaemon), consists of a stem on which are lamella-like plates 
lying upon each other like the leaves of a book. Such a form 
is known as a phvllobranch, and its derivation is seen in 
the individual development of Penaeus (Claus). Here the pri- 
mary rays lengthen and grow round on the side away from the 
body, so as to enclose a sort of canal running parallel to the 
shaft and open at the ends. Secondary rays ariseonthe outer side 
of the primary ones, i. e. projecting into the canal, in a single 
row turned towards the base of the gill. These may split so 
as to appear as though they sprang separately. The flattening 
of these secondary rays into leaf-like plates and the enlarge- 
ment of the primary ones would give rise to a phyllobranch. 
According to phylogenetic development the podobranch is 
the most ancestral of the gills, and it is therefore, at first 
sight, surprising that in the adult Penaeus (a form which is so 
very typical of the whole Decapod group) no trace of podo- 
branchs is to be found, while the pleurobranchs are well de- 
veloped. Looking, however, to the larva of Penaeus, we see 
that podobranchial rudiments are developed (fig: 6), and, 
indeed, they are the first to develop, while the rudiments of 
pleurobranchs ( d ) develop in a later larval stage than all the 
others (fig. 6'). Whilst, however, the arthrobranch and pleuro- 
branchs (6, c, d) go on developing the podobranchial rudi- 
ments (c) disappear, the epipodites, which develop later, being 
