ANNULOSA : CRUSTACEA. 



303 



generally compound, but sometimes simple, and are placed on 

 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^EMODIPODA. Small Crustaceans, with a rudi- 

 mentary abdomen, the first two segments of the thorax amalga- 

 mated with the head, and carrying legs. Branchice as two or three 

 pairs of vesicles, borne on the thorax. The Latmodipoda are small 

 Crustaceans, which are distinguished amongst the Edrioph- 

 thalmata 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. 153); hence the name 

 given to the order. The 

 respiratory organs are in 

 the form of two or three 

 pairs of membranous ves- 

 icles attached to the seg- 

 ments of the thorax, or to 



the bases Of the legS. The Fig. 153. Laemodipoda. Caprellaphasma. 



last pair of feet are either 

 inserted at the end of the last somite, or are followed by not 

 more than one or two small segments. There are four seta- 

 ceous antennae, and the mandibles are without palps. The 

 body is generally linear, of eight or nine joints, but is some- 

 times oval. The feet are hooked. The Lamodipoda are all 

 marine, and one section of the order comprises parasitic Crus- 

 taceans, of which the Whale-louse (Cyamus cefi) 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 L&modipoda, the heart has the form of a long tube 

 extending through the six segments following the head, and 

 having the blood admitted to its interior by three pairs of valv- 

 ular fissures. The three posterior pairs of abdominal limbs 

 are bent backwards, and form, with the telson, a natatory or 



