MANUAL OF NATURAL HISTORY. 279 



short, rounded ; abdomen segmented ; legs 

 elongated. 



III. S UB-CLASS. — Aporo branchiate- Arachnidans 



(Aporobrancliiata). 



Body linear, divided into four distinct segments ; 

 spiracles none ; feet eight, for locomotion, — in the 

 females two false feet for carrying the eggs ; eyes 

 four, smooth, situated on a tubercle. 



1. Family. — True Sea-Spiders (Nymphonidse). Body 



small, elongated ; palpi filiform, furnished 

 with a hooked claw at the end ; legs very 

 long. Free. 



2. Family. — Parasitic Sea-Spiders (Pycnogonidse)- 



Antennae and palpi obsolete ; legs short and 

 robust. Parasitic on Whales. 



CRUSTACEANS. 



These annulose animals differ from the insects in 

 their respiration being performed by means of gills ; 

 their circulation is double, the blood passing through 

 the gills as well as through the body generally; their 

 nervous system forms two ganglionic chains, as in 

 other Annulosa; they have no wings ; and some of 

 their legs are modified to serve as organs of prehen- 

 sion around the mouth. In the Decapods the fore- 

 legs are transformed into arms, with a prehensile 

 pincer at the end, composed of a finger and a move- 

 able thumb, by the help of which strong calcareous 



