SUCTOKIAL CRUSTACEA: ARGULUS. 645 



sites, that their motion and life are soon arrested, apparently 

 because they have not strength to cast-off and renew their enve- 

 lopes. The process of Development appears to depend in some 

 degree upon the influence of Light, being retarded when the 

 animals are secluded from it ; but its rate is still more influenced 

 by Heat ; and this appears also to be the chief agent that regulates 

 the time which elapses between the Moultings of the adult, these, 

 in the Daphnia, taking-place at intervals of two days in warm 

 summer weather, whilst several days intervene between them when 

 the weather is colder. The cast Shell carries with it the sheaths 

 not only of the Limbs and Plumes, but of the most delicate Hairs 

 and Setae which are attached to them. If the animal have pre- 

 viously sustained the loss of a Limb, it is generally renewed at the 

 next moult, as in higher Crustacea.* 



505. Closely connected with the Entomostracous group is the 

 tribe of .Suctorial Crustacea; which for the most part live as 

 Parasites upon the exterior of other animals (especially Fish), 

 whose juices they imbibe by means of the peculiar Proboscis- like 

 organ which takes in them the place of the jaws of other Crus- 

 taceans ; whilst other appendages, representing the Feet-jaws, are 

 furnished with hooks, by which these parasites attach themselves 

 to the animals from whose juices they derive their nutriment. 

 Many of the Suctorial Crustacea bear a strong resemblance, even 

 in their adult condition, to certain Entomostraca ; but more com- 

 monly it is between the earlier forms of the two groups that the 

 resemblance is the closest, most of the Suctoria undergoing such 

 extraordinary changes in their progress towards the adult con- 

 dition, that if their complete forms were alone attended-to, they 

 might be excluded from the class altogether, as has (in fact) been 

 done by many Zoologists. — Among those Suctorial Crustacea which 

 present the nearest approach to the ordinary Entomostracous type, 

 may be specially mentioned the Argulus foliaceus, which attaches 

 itself to the surface of the bodies of Fresh-water fish, and is com- 

 monly known under the name of the ' Fish Louse.' This animal 

 has its body covered with a large firm oval shield, which does not 

 extend, however, over the posterior part of the abdomen. The 

 mouth is armed with a pair of styliform Mandibles ; and on each 

 side of the proboscis there is a large short cylindrical appendage, 

 terminated by a curious sort of Sucking-disk, with another pair of 

 longer jointed members, terminated by prehensile Hooks. These 

 two pairs of appendages, which are probably to be considered as 

 representing the Feet-jaws, are followed by four pairs of Legs, 

 which, like those of the Branchiopods, are chiefly adapted for swim- 

 ming; and the Tail, also, is a kind of swimmeret. This little 



* For a systematic and detailed account of this group, see Dr. Baird's 

 " Natural History of the British Entomostraca," published by the Ray 

 Society. 



