XX THE VOYAGE OF H.M.S. CHALLENGER. 



Sea ; it represents the cephalic neural mass with the oculus or the unpaired eye, with 

 one crystalline sphere in the median line ; but this I have failed to observe in the newly- 

 hatched specimen as shown iD fig. 1 on the same plate, which was obtained direct from 

 the ovum. 



It aj}pears, therefore, that when present, as it is frequently in the Macrura until 

 the animal is well advanced in development, it only exists as the remains of a worn-out 

 organ that belonged to an earlier condition of life, and which only attains its true 

 characters in those animals that produce the brephalos in the Nauplius stage. This 

 unpaired organ appears therefore to be, as Dr. Hartog says, analogous to those existing 

 in the lower forms of life, such as the Planaria, and perhaps also may be compared with 

 those found in the mantle of Pecten and in the tissues of Annelids. 



They are not in any way homologous with those eyes that in the Crustacea 

 are projected on each side of the first somite of the cephalon, and 

 in the Macrura are placed at the extremity of a two- or three- 

 jointed appendage as may be seen in Fig. V. and also in PI. XIV. 

 fig. 2, in Eretmocaris longicaulis and other species on the same 

 plate, in which the organ of vision is projected on an appen- 

 fig. v.-ophtiiaimopod of dao-e of two or three articulations, so that in Eretmocaris it 



Pleswnika uniproducta. c 



considerably resembles the appearance of an antenna that has 

 the extremity modified for the purposes of vision, just as the antennae on the homo- 

 types of other limbs are modified for the purpose of touch, hearing, and smell. 



In Eretmocaris the ophthalmopoda, as well as the first, and perhaps the second 

 antennas, are attached to, and appear to originate in, a lobe that is anterior to and distinct 

 from the carapace, and which also supports the central oculus. 



Tlie First Antennae. — The first antennae form the second pair of appendages, and 

 belong to the second theoretical somite ; but this somite is seldom recognisable as a 

 distinct part, except in the Squilliform Crustacea, and to a less extent, as well as in an 

 aberrant condition, in the Palinuridae. 



The late Professor Milne-Edwards, as a convenient means of defining the first from 

 the second pair of antennas, gave to the anterior the name of antennules, which 

 many authors have adopted, but whicli 1 have not employed in this Report, because the 

 numerical system appears to be both more consistent and of greater value, and the term 

 is suggestive also of diminutiveness or inferiority. Generally the first antennas is 

 proportionally smaller than the second, but usually it is a highly organised structure, 

 and increases in functional power as it diminishes in length. 



The peduncle consists of three joints which terminally support two long and 

 slender flagella, the outer of which must be regarded as of more importance than the 

 inner, for it carries certain organs that are apparently essential to the welfare of the 



