046 ARGULUS; LERNCEA. CTRRHTPEDS. 



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 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. 326). 

 The proper Alimentary Canal, however, 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 marked 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 tissues 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. 331, 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 their 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 

 Microscopic study of their Development.* 



506. From the parasitic Suctorial Crustacea, the transition is 

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

 of C'm'hipeda, 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 true 

 Crustacea. The departure from the ordinary Crustacean type in 

 the adults is, in fact, so great, that it is not surprising that Zoolo- 

 gists in general should have ranked them in a distinct Class ; their 

 superficial resemblance to the Mollusca, indeed, having caused most 

 Systematists to place them in that series, until due weight was 

 given to those structural features which mark their Articulated 

 character. We must limit ourselves, in our notice of this group, 

 to that very remarkable part of their history, the Microscopic 



* As the group of Suctorial Crustacea is rather interesting to the pro- 

 fessed Naturalist than to the amateur Microscopist, even an outline view 

 of it would be unsuitable 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. 



