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 maxillae 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 Tahtrus, 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 



