526 ALLIS. [Vou. II. 
tion of the regular method. They lie relatively close together, 
and the pits in which they lie become a continuous groove, as 
shown in Fig. 12, before the canal closes over either of them. 
The canal is formed as usual, but a relatively long section of it 
is inclosed at once so that the two half-pores which go to make 
pore 16 are formed facing each other and already partly fused. 
The different steps in this process are shown in Figs. 23, 24, 
and 25, in the last of which pore 16 is fully developed. This 
abbreviated method occurs wherever the organs lie close to- 
gether, or where the canals are relatively large, as in young 
larvee. 
In Figs. 23 and 24, organ 8 supra-orbital lies on the surface, 
and is hardly to be distinguished except by position from the 
following organs of the anterior dorsal pit line. In Fig. 25, — 
which represents a 234-millimetre specimen, it is just being 
inclosed, and in Fig. 27, a 29-millimetre one, is entirely so, pore 
8 appearing as a small terminal opening immediately in front of 
the first organ of the pit line. 
In Fig. 23, organs 14 and 15 infra-orbital can both be seen; 
in Fig. 24 they are just inclosed ; and in the succeeding figures 
up to 28 the principal phases in the formation of pore 15 are 
shown. This pore lies at a sharp bend in the infra-orbital line 
close to pore 7 supra-orbital, which also lies at a bend in its line. 
The two lines are approaching each other at these bends, and 
the surface between them is strongly depressed, just as it is be- 
tween successive organs on a regularly developing line. Along 
this depression pores 15 and 7, once formed, approach each 
other and coalesce, exactly as half-pores do along the line of a 
canal. In Figs. 29 and 30 they are shown partly united, and in 
Fig. 31, a 40-millimetre specimen, they have already united and 
begun their primary division, which is completed in Fig. 33, a 
54-millimetre specimen. In Fig. 34, a specimen 135 millimetres 
long, the double pore has undergone its secondary division, and 
is represented by a group of four. 
In Figs. 23 and 24, organ 16 operculo-mandibular and all the 
organs of the infra-orbital line behind organ 16, lie on the sur- 
face or in small pits along the strong channels which mark the 
course of the canal. In Fig. 25 the canal is closing over organs 
17, 19,and 20. In Fig. 26 it has fully closed over organs 17 and 
19, but organ 20 is still visible in a large double pit common to 
