Lepeophtheirus dissimulatus — Lewis 
201 
slightly less than the width of the anterior end 
in younger specimens. The third pedigerous seg- 
ment bears a single naked, spinelike process 
projecting from each of the posterior lateral 
corners (Fig. 12^). The relationship of this 
process to the future third thoracic leg is un- 
known although Heegaard (1947:63, fig. 8) 
figures a pair of lobate appendages in the same 
region as the spines and indicates that they be- 
come the third thoracic legs of Caligus curtus. 
The free fourth pedigerous segment is short, its 
greatest width being almost 3 times its length. 
The anterior margin is concave, the posterior 
convex. There is no indication of the fourth 
thoracic legs. 
The free fourth segment, the future genital 
segment and abdomen, is slightly shorter than 
the combined lengths of the third and fourth 
pedigerous segments, and the width is slightly 
greater than the length. The lateral margins are 
flatly convex, and the lateral posterior margins 
are concave, forming the attachment surface, 
and possibly an articulation surface, for the 
caudal rami. The posterior medial margin of the 
segment is bilobed, and the anal region is evi- 
dent as a minute, V-shaped depression between 
the lobes. 
The copepodite possesses a spikelike rostrum 
on the anterior end of the cephalothorax (Fig. 
13 a). The frontal organ is now visible, although 
rather indistinctly, and is represented by two 
large ovoid bodies at the anterior end of the 
cephalothorax that are connected to the rostrum 
by a slender, tubelike process. The alimentary 
tract is visible as two convex lines extending 
from just behind the eyes posteriorly for ap- 
proximately one-third the length of the cepha- 
lothorax. The tract is visible posterior to the 
two convex lines only as a slender mass of yolk 
globules along the median longitudinal axis and 
extending posteriorly to the free third pediger- 
ous segment. There does not appear to be an 
oral opening. 
The length of time spent in the copepodite 
stage is not known due to the inability of get- 
ting a copepodite to attach to a substrate and 
moult into the first chalimus in the laboratory. 
In order to determine the method of attachment, 
observations were made on specimens of copep- 
odites attached to host material collected in the 
field. The copepodite attaches to the host by 
means of the second antennae, secretes a frontal 
filament, and then after a short period of time 
moults into the first chalimus. This process is 
described in detail in the portion of the paper 
devoted to the general behavior of the develop- 
mental stages. The only visible difference be- 
tween the free-swimming and attached copep- 
odites is the slightly larger frontal organ in 
the attached forms. All of the appendages and 
processes and the general body structure are 
identical. Heegaard (1947:53-65), however, 
describes two copepodite stages for Caligus 
curtus, the first copepodite moulting into the 
second shortly after the host is found. In addi- 
tion he describes a "pupal stage” that is passed 
within the loosened but unshed second copep- 
odite cuticle. Neither the second copepodite 
stage nor the pupal stage were found in Lepe- 
ophtheirus dissimulatus. 
The change from the copepodite to the adult 
is gradual for the most part. There are excep- 
tions to this in some of the appendages such as 
the first and second thoracic legs but, in general, 
the transition occurs in a step-by-step manner 
through the six chalimus stages which are found 
between the copepodite and the adult. 
The first chalimus (Fig. 8g), attached to the 
host by the frontal filament, is dorsoventrally 
flattened. The body is ovoid in a dorsal view 
and the greatest length, including the frontal 
region but not the frontal filament, is approxi- 
mately 1.2 times the greatest width. The cepha- 
lothorax consists of the cephalon, maxilliped- 
bearing segment, and first pedigerous segment as 
in the copepodite. The second, third, and fourth 
pedigerous segments, in addition to the single 
segment that will later form the genital segment 
and abdomen, are free. The anterior margin of 
the cephalothorax is broadly curved except for 
an irregular median swelling in the region of 
the frontal filament. The lateral cephalothoracic 
margins flare slightly anteriorly, just posterior 
to the antennule base, and then curve flatly pos- 
teriorly. The posterior sinuses are distinct but 
are formed of the anterior lateral margin of the 
second pedigerous segment and the inner mar- 
gin of the posterior lateral cephalothoracic area, 
not the outer margin of the median cephalotho- 
racic area and the inner margin of the posterior 
lateral cephalothoracic area as in the copepodite. 
The median margin of the thoracic area is flatly 
