1904.] ASELLOTA-GBOUP OF CRUSTACEANS. 313 



in the male of other Isopoda. In Idothea, Sphceroma, Cirolana, 

 Cymothoa, (fee, this endopod is generally described as an undivided 

 plate with an "'appendix masculina '"' articulated at its inner 

 margin: this plate is thefii-st, the ^'■appendix'" the second joint of 

 the endopod. This endopod is therefore two-jointed in all Isopoda, 

 Epicaridea and Gnathiidfe excepted; but in most forms only the 

 second joint is transformed, the first being large and lamellar like 

 that of the first or the third pair, while in Asellota and Oniscidfe 

 both joints are narrow. 



VI. The Classification of the Asellota. 



The tribe or suborder Asellota is very sharply defined from 

 all other Isopoda, but its subdivision into families is a matter of 

 considerable difficulty. As already stated, G. 0. Sars in 1897 

 divided the Asellota into five families, but four of these are 

 far from distinct from each other ; moreover, other objections 

 can be raised. His family Desmosomatidee is in reality a rather 

 mixed company : such genera as Macrostylis and Ischnosoma 

 differ strongly from each other in most features ; Rannoniscus 

 and especially Ischnosoma are far from being closely related to 

 Besinosonia, &c. I have, for the rest, already, on p. 308, pointed 

 out several difficulties as to these four families ; it may be added 

 that from the ' Ingolf ' we have several new forms which differ 

 rather or very considerably from the genera of Sars, so that an 

 attempt at arranging them within his families will aggravate the 

 state of things. When nature has not worked out groups well- 

 defined from each other we can of course subdivide a tribe or 

 suborder into families, founding them on some points in the general 

 aspect of the animals, but their number and limitation must 

 then be a matter of personal opinion, and many other authors will 

 propose the establishment of other or of new families not better 

 than those first erected. It is, in my opinion, to be jtreferi^ed to 

 keep a very large group of genera in the same family, a large number 

 of species in the same gemcs, than to subdivide res2)ectively the family 

 or genus into families and genera with new names, %vhen sharp 

 lines of distinction are not to be found in nature. 



It is well-known that dififerences in the structure of the 

 abdominal appendages are among the most important characters 

 for dividing the order Isopoda into tribes or main- families. That 

 considerable importance must be ascribed to the above-named 

 differences in the structure of the pleopoda in Aselhcs, Stenetritom, 

 and other Asellota, will probably be admitted, these differences 

 being much sharper than those met with in any other external 

 organ. In the Plankton paper I wrote in 1895 the long passage 

 quoted above on differences in the pleopoda between Asellus and 

 Stenetriwm on one side, and several other genera of Asellota 

 on the other, and continued : — " Es ist anzunehmen, dass alle 

 existirenden Gattungen in die eine oder dieandere dieser zwei nach 

 ausserst scharfen Kennzeichen getrennten Gruppen eingefiigt 

 werden konnen, welche also die 2 Famiiien bilden, in welche die 



