212 THE MICROSCOPE AND ITS REVELATIONS . 



seen to be so covered with such parasites, that their motion and life are 

 soon arrested, apparently because they have not strength to cast-off and 

 renew their envelopes. The process of development appears to depend 

 in some degree upon the influence of light, being retarded when the ani- 

 mals 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 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 previously sustained tne loss of a limb, it is generally renewed at 

 the next moult, as in higher Crustacea. 1 



611. 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 Crustaceans; 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 commonly 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 condition, 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 Entomos- 

 tracous type, may be specially mentioned the Argulus foliaceus, which 

 attaches itself to the surfaces of the bodies of fresh-water Fish, and is 

 commonly 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 

 swimming; and the tail, also, is a kind of swimmeret. This little animal 

 can leave the fish upon which it feeds, and then swims freely in the 

 water, usually in a straight line, but frequently and suddenly changing 

 its direction, and sometimes turning over and over several times in suc- 

 cession. The stomach is remarkable for the large caecal prolongations 

 which it sends out on either side, immediately beneath the shell; for 

 these subdivide and ramify in such a manner, that they are distributed 

 almost as minutely as the caecal prolongations of the stomach of the 

 Planaria (Fig. 406). The proper alimentary canal, however, is con- 

 tinued backwards from the central cavity of the stomach, as an intestinal 

 tube, which terminates in an anal orifice at the extremity of the abdo- 

 men. A far more marked departure from the typical form of the class 



1 For a systematic and detailed account of this group, see Dr. Baird's " Natu- 

 ral History of the British Entomostraca," published by the Eay Society. 



