TIII-: i;i!.\nivruA. 299 



auditory ossicle ; but, as in Aatacits, the auditory sacs are, 

 in fact, lodged in the dilated basal joint of the antennule. 



A cervical fold, lodging- the scaphognathite, occupies the 

 same relative position as in Astacus, and marks off the 

 cephalic form of the thoracic region, on the sides of the body. 

 The thoracic sterna gradually increase in breadth, and the 

 posterior ones are marked externally by a strong median, 

 longitudinal depression, answering to a corresponding fold 

 on the inner surface. The apodemal cells are well formed, 

 but the sternal canal, so largely developed in the Macrura, is 

 absent in this, as in all other tirachyura. 



The structure of the appendages is essentially the same 

 as in Astacus, but the third thoracic appendage, or external 

 maxillipede, has its ischiopodite and meropodite greatly en- 

 larged, so as to form a broad plate, which, with its fellow, 

 covers over the other organs, and hence receives the name of 

 the gnathostegite. The three terminal joints of the limb re- 

 main small, and constitute a pulpiform appendage the en- 

 dognathal palp. 



In some of the lower Macrura the thoracic limbs are pro- 

 vided with a short exopodite, and the posterior maxillipedes 

 become undistinguishable from the ordinary thoracic limbs. 

 Such forms lead us naturally to the Schizopoda, a group the 

 name of which is derived from the apparent splitting of the 

 limbs produced by the great development of the exopodite, 

 which, in these Crustacea, is as large as the endopodite. In 

 this group, again, a line can hardly be drawn, in many cases, 

 between any of the mnxillipedes and the thoracic limbs, the 

 anterior pair only being somewhat smaller than the rest. 

 Hence Tkysanopoda is admitted, by Milne-Edwards, to have 

 eight pairs of thoracic limbs (" Crustaccs," ii. 464). The 

 branchiae in the Schizopoda are frequently absent ; when 

 well developed, as in Thysanopoda, they are not included 

 under the branchiostegite, but hang down freely from the 

 bases of the thoracic limbs. In My sis, the only represen- 

 tative of a branchia (if it be one in reality) is a process at- 

 tached to the first thoracic appendage. Cynthia has its 

 branchial appendages attached to the abdominal members. 



In TJiysanopoda, Mysis, and Cynthia, the general struct- 

 ure of the body is similar to that of the Macrur<i, except 

 that, in Mysis, the greater number of the abdominal append- 

 ages are rudimentary. 



In Leucifer, the antennary somite is produced into a very 

 long and narrow peduncle, which supports the eyes, on their 



