548 CRUSTACEA. 



gether, 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 foUaceus, which attaches itself to the surface of the 

 bodies of fresh-water fish, and is commonly known under the 

 name of the "fish-louse." This animal has his 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 pro- 

 boscis 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 repre- 

 senting the feet-jaws, are followed by four pairs of legs, which, 

 like those of the Branchiopoda, are chiefly adapted for swim- 

 ming ; and the tail, also, is a kind of swimmeret. This little ani- 

 mal can leave the fish on which it feeds, and then swims freely 

 in the water, usually in a straight line, but frequently and sud- 

 denly changing its direction, and sometimes turning over and over 

 several times in succession. The stomach is remarkable for the 

 large caecal prolongations which it sends out on either side, im- 

 mediately beneath the shell ; for these subdivide and ramify in 

 such a manner, that they are distributed almost as minutely 

 as the caBcal prolongations of the stomach of the Planaria (Fig. 

 273). The proper alimentary canal, however, is continued back- 

 wards from the central cavity of the stomach, as an intestinal 

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

 abdomen.' 



373. From the parasitic suctorial Crustacea, the transition is 

 not really so abrupt as it might at first sight appear, to the class 

 of Cirrhipeda, consisting of the Barnacles and their allies ; which, 

 like many of the Suctoria, are fixed to one spot during the adult 

 portion of their lives, but come into the world in a condition 

 that bears a strong resemblance to the early state of many of the 

 Crustacea. The departure from the ordinary Crustacean type in 

 the adult, is, in fact, so great, that it is not surprising that Zoo- 

 logists in general should have separated them ; their superficial 

 resemblance to the Mollusca, indeed, having caused most syste- 

 matists to rank them in that series, until due weight was given 

 to those structural features which mark their Articulated charac- 

 ter. We must limit ourselves in our notice of this group, to that 

 very remarkable part of their history, the Microscopic study of 

 which has contributed most essentially to the elucidation of tifieir 

 real nature. The observations of Mr. J. V. Thompson,^ with the 

 extensions and rectifications which they have subsequently re- 



' As this group is rather interesting to the professed NaturaUst than to the amateur 

 Microscopisf, even an outline view of it would be unsuitable to the present treatise ; 

 and the Author would refer such of his readers as may desire to study it, to the ad- 

 mirable treatise by Dr. Baird, already referred to. 



2 ■' Zoological Researches," No. Ill, 1830. 



