966 CRUSTACEA 



progress towards the adult condition that, if their complete forms 

 were alone attended to, they might be excluded from the class 

 altogether, as was (in fact) done by many earlier zoologists. Of the 

 suctorial Crustacea which form the group Branchiura may be 

 specially mentioned the Argulus foliaceus, which attaches itself to 

 the surface of the bodies of fresh-water fish, such as the stickleback, 

 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 ap- 

 pendage, terminated by a curious sort of sucking-disc, 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 foot-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 succession. The stomach is remarkable for the large csecal 

 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 Planar ia (fig. 714). The proper alimentary canal, how- 

 ever, is continued backwards from the central cavity of the stomach, as 

 an intestinal tube, which terminates in an anal orifice at the extremity 

 of the abdomen. A far more remarkable departure from the typical 

 form of the class is shown in the Lerncea, which is found attached 

 to the gills of fishes. This creature has a long suctorial proboscis ; 

 a short thorax, to which is attached a single pair of legs, which meet 

 at their extremities, where they bear a sucker which helps to give 

 attachment to the parasite ; a large abdomen ; and a pair of pendent 

 egg-sacs. In its adult condition it buries its anterior portion in the 

 soft tissue of the animal it infests, and appears to have little or no 

 power of changing its place. But the young, when they come forth 

 from the egg, are as active as the young of Cyclops (fig. 720, C. D), 

 which they much resemble ; and only attain the adult form after a 

 series of metamorphoses, in which they cast off their locomotive 

 members and eyes. It is curious that the original form is retained 

 with comparatively slight change by the males, which increase but 

 little in size, and are so unlike the females that no one would suppose 

 the two to belong to the same family, much less to the same species, 

 but for the study of their development. 1 



From the parasitic suctorial Crustacea the transition is not 



1 As the group of suctorial Crustacea is interesting rather to the professed 

 naturalist than to the amateur microscopist, even an outline view of it would be un- 

 suitable to the present work ; and the Author would -refer such of his readers as may 

 desire to study it to the excellent treatise by Dr. Baird already referred to. Of the 

 numerous recent essays and memoirs those of Professor Glaus should by all means 

 be consulted. Mr. P. W. Bassett-Smith, Staff-surgeon E.N., has in the last few years 

 published several interesting papers. 



