A STUDY IN MORPHOLOGY. 
97 
The history of this appendage in Lucifer shows that that there is no reason, except 
the arbitrary system borrowed from the higher Decapods, for classing this appendage 
with the mouth parts, instead of with the thoracic limbs. 
It appears much later than the first and second pairs of maxillipeds, or at the same 
time with the thoracic limbs. It agrees with these latter in all its subsequent changes 
and in its adult structure, and must be regarded as forming one of the thoracic series. 
I have employed the recognised name, third maxilliped, to prevent confusion, but the 
appendage is in no sense a mouth part. In fact, the only reason for holding that the 
missing appendage in Lucifer is the fifth pereiopod, instead of the last maxilliped, is 
the tacit assumption that the appendages must follow a definite serial order from in 
front backwards. We do not know that this assumption is justifiable in all cases, and 
it is therefore perfectly possible that the appendage which is usually called the third 
maxilliped of Lucifer may really be the first pereioped. I think the probability is in 
favour of the accepted homology, but the use of the term “ third maxilliped ” in the 
present paper for the appendage in question must not be regarded as evidence that 
the homology is accepted without question. 
The pereiopods. 
At the end of the Zoea series four pairs of pereiopods, the first, second, third, 
and fourth, are represented by buds (figs. 43 and 45), while the fifth is entirely absent, 
as Dana pointed out in the ‘ Report on the Crustacea collected by the United States 
Exploring Expedition,’ p. 634. Willemoes-Suhm (Proc. Roy. Soc., vol. 24, p. 134), 
calls attention to the same fact : the total absence of this somite at all stages of 
development. In the Schizopod stage each of these appendages is biramous (fig. 59), 
and similar to the last maxilliped, although the first three pairs (fig. 59, Pr. 1, Pr. 2, 
and Pr. 3) are longer. 
At the end of the Schizopod series of stages the entire fourth pair and the exopo- 
dites of the other three pairs disappear, and the endopodites lengthen to form the 
long slender limbs of the adult (fig. 61, Pr. 1, Pr. 2, and Pr. 3). They are four- 
jointed, with a double row of short hairs along the anterior edge, and the first is only 
half as long as the second and third, which are nearly equal, and almost as long 
as the carapace and neck. The third ends in a short, curved hairy claw, too small 
to be shown in the figure. 
The first abdominal appendage. 
This is present as a rudimentary bud at the end of the Schizopod series, but does 
not become functional until the Lucifer form is reached. In the young it consists 
of a long unjointed base, and a single pointed tip, fringed with swimming hairs 
(fig. 61, PI. 1). In older specimens the basal portion divides into two joints, and in 
the young male or the young or mature female the appendage has the form shown 
in fig. 74. As the male approaches maturity a small process, shown in fig. 76, 
MDCOCLXXXII. o 
