CLASSIFICATION OF CRUSTACEA. H 



In the other, the Isopoda, the abdomen is short, with the articula- 

 tions admitting of little flexion ; four or five pairs of abdominal appen- 

 dages are lamellar and branchial, and only the posterior pair styliform. 



Such are the characteristics laid down respectively for these groups. 

 But there is another character of high importance, which has not been 

 alluded to by previous authors, corroborative of this arrangement of 

 the species. In the Amphipoda, four pairs of the thoracic feet are 

 directed forward, and three outward and backward: while in the Iso- 

 poda three pairs are directed forward, and four backward; that is, the 

 sets of legs are four anterior and three posterior in the Amphipoda ; 

 three anterior and four posterior in the Isopoda. 



A third subdivision of the Choristopoda was proposed by Latreille, 

 under the name Loemipoda (or Loernodipoda) . The species included 

 are characterized by having the abdomen nearly or quite rudimentary. 

 In the more essential characters they are closely related to the Amphi- 

 poda, rather than to the Isopoda, and are not properly intermediate, 

 nor a new type alike distinct from both ; for they have the thoracic 

 branchial vesicles of the Amphipoda, and the abdomen in species that 

 have this part somewhat elongated, partakes of the Amphipod charac- 

 ter. They are properly therefore Amphipoda, with certain parts obso- 

 lescent. That this is a correct view of their relations is shown by the 

 thoracic feet, the four anterior pairs being forward feet, as in the Am- 

 phipods. This conclusion has been adopted by many Zoologists. 



There are, however, true intermediate species between the Amphi- 

 pods and Isopods, and if any third or intermediate group is admitted, 

 these should be considered as constituting it. These species belong to 

 the genera Tanais, Arcturus, Leachia, and others allied. Like the 

 Amphipoda, they have the four anterior pairs of feet of the forward 

 series, and the three posterior of the hinder ; but like the Isopoda, 

 they have the abdomen very short, and composed of six very short 

 joints, and only the last pair of members is styliform (instead of the 

 three posterior pairs, as in normal Amphipoda), while the others are 

 lamellar and branchial, as in Isopods. 



We therefore recognise three groups or tribes of the Choristopods, as 

 follows : 



1. Amphipoda. Branchial vesicles thoracic; forward series of tho- 

 racic feet eight in number. 



2. Anisopoda. Branchial vesicles abdominal ; forward series of tho- 



