A STUDY IN .MORPHOLOGY. D7 



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 Jirst 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 uujointed base, and a single pointed tip, fringed with swimming hairs 

 (fig. 61, PL 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, 



MDCCCLXXXII. o 



