CRUSTACEA. 213 



is shown in the Lernaa, 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. 411, c, D), which they much resemble; and only attain the adult 

 form after a series of metamorphoses, in which they cast-off their loco- 

 motive 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 

 Microscopic study of their development. 1 



612. From the parasitic Suctorial Crustacea, the transition is not 

 really so abrupt as it might at first sight appear to the group of Cirrhi- 

 peda, consisting of the Barnacles and their allies: for these, like many of 

 the Suctoria, arc 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 Zoologists 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 ( articu- 

 lated ' character. 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 their real 

 nature. The observations of Mr. J. V. Thompson, 2 with the extensions 

 and rectifications which they have subsequently received from others 

 (especially Mr. Spence Bate 3 and Mr. Darwin 4 ) show that there is no 

 essential difference between the early forms of the sessile (Balanidae or 

 'acorn-shells ') and of the pedunculated Cirrhipeds (Lepadidaeor ' barna- 

 cles'); for both are active little animals (Fig. 412, A), possessing three 

 pairs of legs, and a pair of compound eyes, and having the body covered 

 with an expanded carapace, like that of many Entomostracous Crusta- 

 ceans, so as in no essential particular to differ from the larva of Cyclops 

 (Fig. 411, c). After going through a series of Metamorphoses, one 

 stage of which is represented in Fig. 412, B, c, these larvas come to pre- 

 sent a form, D, which reminds us strongly of that of Daphnia; the body 

 being inclosed in a shell composed of two valves, which are united along 

 the back, whilst they are free along their lower margin, where they sepa- 

 rate for the protrusion of a large and strong anterior pair of prehensile 

 limbs provided with an adhesive sucker and hooks, and of six pairs of 

 posterior legs adapted for swimming. This Bivalve shell, with the 



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

 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. 



1 "Zoological Researches," No. iv., 1830, and Philos. Transact., 1835, p. 355. 



3 ' On the Development of the Cirripedia,' in " Ann. of Nat. Hist.," Ser. 2, Vol. 

 viii. (1851), p. 324. 



4 " Monograph of the Sub- Class Cirripedia," published by the Ray Society. 



