204 
tions and form the middle of the posterior sur- 
face of the segment. 
The frontal organ of the second chalimus 
(Fig. 1 3c) is similar to that of the first chalimus 
although more distinct. The lobes at the proxi- 
mal end of the frontal filament are contiguous 
along the median longitudinal axis of the body, 
forming a single ovoid structure; the lobes pos- 
terior to the first set are slightly smaller than 
in the preceding stage. The alimentary tract 
now appears complete although the anterior end 
is still filled with yolk granules. 
The 10 specimens maintained successfully in 
the laboratory, at room temperature (approxi- 
mately 23 C), remained in the second chalimus 
for about 42 hr. 
The third chalimus (Fig. 9c) is somewhat 
wider than the second. The greatest length, in- 
cluding the frontal region but not the frontal 
filament, is approximately 1.3 times the greatest 
width. The cephalothorax now includes the first 
three pedigerous segments, the condition occur- 
ring in the adult. The fourth pedigerous seg- 
ment is free and forms the connection between 
the cephalothorax and the now indistinctly dif- 
ferentiated genital segment and abdomen. The 
frontal region (Fig. 13 d) is still indistinctly 
separated from the cephalothorax. The anterior 
margin of the frontal region is flatly tapered 
from the projection of the frontal filament lat- 
erally. The lateral portions of the frontal mar- 
gin are evenly rounded and join the lateral 
margins of the cephalothorax at a slight inden- 
tation marking the origin of the groove in- 
completely dividing the frontal region from the 
cephalothorax. The lateral margins of the cepha- 
lothorax are smoothly convex, the posterior lat- 
eral corners rounded. The posterior sinuses are 
distinct and U-shaped; the lateral margins of 
the median thoracic area are continuous with 
the inner margins of the sinuses. The median 
thoracic area extends posteriorly past the poste- 
rior extensions of the lateral areas. The reddish- 
pigmented regions of the eyes encircle all but 
the outer portion of the lenses, a large increase 
in the amount of pigmented material over the 
preceding stages. The cephalothoracic grooves 
are distinct, the major ones forming an H. The 
longitudinal grooves of the H extend anteriorly 
and curve medially from their origin just lateral 
to the apices of the posterior sinuses. The two 
PACIFIC SCIENCE, Vol. XVII, April 1963 
grooves are connected by an anteriorly curving 
cross groove in their anterior region. Other in- 
distinct grooves are present in the anterior re- 
gion of the cephalothorax. 
The free fourth pedigerous segment is short 
and partially covered by the posterior portion 
of the median cephalothoracic area. The greatest 
length of the segment is approximately one-half 
the greatest width. The lateral margins are con- 
vex; the posterior margin is distinct, completely 
separating the segment from the genital seg- 
ment. 
The genital segment (Fig. 12^) is short, the 
length slightly more than one-half the greatest 
width, approximately equal to the length of the 
fourth pedigerous segment. The lateral margins 
are convex anteriorly and indented posteriorly. 
The posterior margin is light but distinct, and 
the abdomen is now considered distinct from 
the genital segment. A pair of one-segmented 
dactyliform processes, the fifth legs, are visible 
extending posteriorly from the posterior lateral 
surface of the genital segment. The distal end 
of the fifth leg is rounded and bears two plu- 
mose setules. 
The abdomen (Fig. 12 d) is short, the great- 
est length slightly less than that of the genital 
segment, the greatest width approximately 2Vi 
times the length. The lateral margins are con- 
vex and the posterior margins curve medially to 
the bilobed anal region, composed of the anal 
laminae. 
The frontal organ (Fig. 13 d) is slightly 
smaller than that of the second chalimus and 
consists of four parts. The first part is formed 
of two oval bodies at the base of the frontal 
filament, one on either side of the median longi- 
tudinal axis, that are contiguous along their in- 
ner margins. A somewhat larger circular body 
is present, attached to the posterior surface of 
the two anteriormost bodies. An elongate, bi- 
furcate body is attached to the posterior surface 
of the circular body; both distally rounded parts 
of the bifurcation extend posteriorly and slightly 
laterally into a larger semirectangular fourth 
part. The change in the shape of the organ from 
the preceding chalimus may be due to a shrink- 
ing of the structure, the anterior two parts being 
formed from the single ovoid structure present 
on the second chalimus; the semirectangular 
fourth part can not be traced to any definite 
