224 MANUAL OF ZOOLOGY. 



the sides of the head. The head is almost always distinct 

 from the body, and the mandibles are often furnished with a 

 palp. Typically there are seven pairs of feet in the adult, 

 hence this division is called Tetradecapoda by Agassiz. In 

 certain Isopods (Tanais) alone is there a carapace. 



Order I. L^modipoda.- — The Lcemodipoda are small Crus- 

 taceans, which are distinguished amongst the Edriophthalmata 

 by the rudimentary condition of the abdomen. The first 

 thoracic segment is amalgamated with the head, and the limbs 

 of this segment appear to be inserted beneath the head, or, as 

 it were, beneath the throat (fig. 71); hence the name given to 

 the order. The respiratory organs are in the form of two or 

 three pairs of membranous vesicles attached to the segments 



of the thorax, or to the 

 bases of the legs. The 

 last pair of feet are either 

 inserted at the end of the 

 last somite, or are followed 

 by not more than one or 

 Fig. 71.— LKmodipoda. Cafreiia phasnta. two Small Segments. There 



are four setaceous antennae, 

 and the mandibles are without palps. The body is gene- 

 rally linear, of eight or nine joints, but is sometimes oval. 

 The feet are hooked. The Lcsmodipoda are all marine, and 

 one section of the order comprises parasitic Crustaceans, of 

 which the Whale-louse {Cyamus Ceti) is the most familiar. 

 The entire order is now generally regarded as being merely 

 a section of the Amphipoda. 



Order II. Amphipoda. — The members of this order re- 

 semble those of the preceding in the nature of the respiratory 

 organs, which consist of membranous vesicles attached to the 

 bases of the thoracic limbs. The first thoracic segment, how- 

 ever, is distinct from the head, and the abdomen is well 

 developed, and is composed of seven segments. There are 

 seven pairs of thoracic limbs, directed partly forwards, and 

 partly backwards, the name of the order being derived from 

 this circumstance. As in the Lcemodipoda, the heart has the 

 form of a long tube extending through the six segments follow- 

 ing the head, and having the blood admitted to its interior by 

 three pairs of valvular fissures. The three posterior pairs of 

 abdominal limbs are bent backwards, and form, with the tel- 

 son, a natatory or saltatorial tail. The young Amphipod 

 acquires its full number of segments and limbs before its 

 liberation from the egg ; and as a rule the young undere;o 

 little or no metamorphosis in reaching -maturity. 



