TANAID^. 117 



Tribe— VACANTIA. 



The non -parasitic aberrant Crustacea are distinguished 

 from the sub-parasitic ones not only by their different 

 condition of life but also by their having fully developed 

 gnathoj)oda (and consequently possessing seven pairs of 

 legs), of which, at least, the anterior pair is of a large 

 size and more or less cheliferous. 



They form two families, Tanaidce and Anthuridce. 



Fam.— TANAID^. 



The body in these Isopods is long and narrow. 

 The head is confluent with the first segment of the 

 body, the eyes are more or less distinctly placed on 

 footstalks. The antennae vary in length. The first 

 pair of gnathopoda, or the anterior pair of legs, are 

 of large size and cheliferous, or terminated by a didac- 

 tyle claw ; the second pair are of an intermediate form, 

 but approximating more nearly to that of the pereiopoda 

 than to that of the first pair. The eggs are borne 

 in a sub-pectoral pouch beneath the five central segments 

 of the body. The tail is terminated by two setaceous 

 articulated appendages. 



The family corresponds with Professor Milne Edwards' 

 tribe of AseUotes lieteropodes — the Aselluta heteropa of 

 Latreille's Families naturelles. It comprises the genera 

 Apseudes {Rhcea), Tanais, Paratanais, and Leptochelia. 



The last-named exotic genus is especially interesting 



