xvi INTRODUCTION. 
it appears to be capable of being slightly moved, and 
probably assists the mandibles in the process of manduca- 
tion. There are three pairs of Siagnopoda, the two ante- 
rior of which are extremely delicate foliaceous appendages, 
whilst the third is much more robust, yet still possessing 
a foliaceous character, particularly as regards the three or 
four basal joints. In some genera, as in Sulcator, some 
of the plates, particularly of the two anterior pairs, are 
folded so as to become two or three parallel leaves, one of 
which, on the first pair in Sulcator, is developed into a 
prominent lobe, containing large nucleated cells. Of the 
office or use of this gland-like organ we can offer no sug- 
gestion, not having met with any analogue in the order. 
The two anterior pairs, the maxille of authors, vary 
somewhat in their form in genera, and very much between 
the Isopoda and Amphipoda. In the parasitic species of | 
both orders, they are defective, and sometimes wholly 
wanting. 
The third Siagnopod, or first maxillipede of authors in 
these orders, is a true cephalic appendage, and covers the 
organs of the mouth as a protecting operculum. 
These last three pairs of appendages are concentrated 
about the mouth, the segments to which they belong being 
represented by the ventral portions only, and these are 
closely fused together, from the sides of which, in the 
genus Talitrus, originate two bony processes, that meet, 
without uniting, near the internal centre of the head, 
there spreading out into flattened plates, from each of 
which a thin and somewhat delicate process is directed 
anteriorly and slightly upwards; the stomach is supported 
by them in its position. This osseous internal arch, that we 
described in the British Association Report “On the British 
Edriophthalma,” 1855, Professor Huxley has, in his lec- 
tures at the Royal College of Surgeons, published in the 
