SUCTORIAL CRUSTACEA ; SIPHONOSTOMA. 275 



XII. ARANEIFORMES. This very curious group, which was 

 formerly ranked among the Arachnida, has the feet adapted for 

 walking, and well developed ; but the mouth is unpossessed of 

 distinct mandibles. 



/.. 



FIG. 498. ARGULUS FOLIA- 

 CEIIS. 1, the animal mag- 

 nified ; 2, one of the large 

 anterior sucking-feet , 3, 

 the siphon ; 4, natural 

 length. 



ORDER X. SIPHONOSTOMA. 



813. The Crustacea of this Order are all parasitic upon Fishes, 

 aquatic Batrachia, &c. ; and many of them are known under the 

 name of Fish-lice. Their general characters may be understood 

 from those of the two genera Argulus and Caligus, which will 



be briefly described. The Argulus has 

 its body covered with an oval shield, which 

 does not extend, however, over the poste- 

 rior part of the abdomen. There are four 

 pairs of legs, somewhat resembling those 

 of the Branchiopoda, and adapted for 

 swimming. The tail also is a kind of 

 swimmeret. On each side of the beak 

 or proboscis, there is a large short cylin- 

 drical appendage, terminated by a curious 

 sucking-disc ; and there is also a pair of 

 larger jointed members, terminated by prehensile hooks. These 

 two pairs of organs, enabling the animal to attach itself firmly 

 to that on which it lives, are probably the representatives of the 

 feet-jaws. The eggs of the Argulus are deposited upon floating 

 bodies, like those of the Cyclops ; and the young, when they 

 first come forth, bear a strong resemblance to those of that genus. 

 Their swimming organs consist of the oar-like antennas and feet- 

 jaws ; the true swimming legs, and the suckers and hooks on 

 the feet-jaws of the adult, not making their appearance until 

 after several moultings. These Crustaceans may be regarded as 

 in several respects intermediate between those of the preceding 

 Order, and the true Suctorial Crustacea ; for they have, even in 

 the adult state, much more locomotive power than the generality 

 of the latter ; and are not unfrequently met with swimming freely 

 in the water. 



814. The Caligus (Fig. 499) is much more strange in its form 



