148 LECTURE XIII. 



phoses before attaining that state as to mask their true relations, not 

 only to the class, but to the primary division of animals to which 

 they belong. This is especially the case with the creatures whose 

 organisation and development will form the subject of the present 

 lecture. 



This elongated, cylindrical, unarticulated Lernaea* {fig' 78.), 

 g^ whose smooth soft body seems devoid of any other appendages 

 than the two long slender ovisacs, might be regarded as one 

 of the Entozoa of the fish to which it is attached, and on the 

 nutrient juices of which it subsists. 



This barnacle, imprisoned in its conical calcareous shell, 

 and cemented to the stone on which it grew, might seem as 

 naturally to belong, like its neighbour the limpet, to the tes- 

 taceous Mollusca. 

 The most vivid imagination of the boldest generaliser or speculator 

 upon the unity of organisation in the Animal Kingdom could never 

 have divined that the Lernaea and the Cirripede were at one period 

 of their lives locomotive animals, swimming about under very similar 

 forms, and by almost identical natatory instruments ; not under the 

 common ciliated infusorial form, in which the young of certain 

 Entozoa and Mollusca first enter into active life ; but with sym- 

 metrical pairs of jointed setigerous legs like those of the lower 

 organised Crustaceans, to which the Epizoa and Cirripedia are, in 

 fact, essentially and most closely allied, although they end their career 

 as sedentary animals under such different, such diversified, and, as 

 regards the Epizoa, such grotesque forms. 



These metamorphoses lead to very different results from those of 

 the Medusa and Comatula. The Epizoa and Cirripedes acquire 

 increase of bulk and organs of generation ; but, in every other 

 respect, the varied course of their development ends in a retrograde 

 movement. This development would seem to have been at first, as 

 it were, hurried forward at too rapid a pace, and the young parasite, 

 starting briskly into life, ranging to and fro by the highest developed 

 natatory organs we have yet met with, and guiding its course by 

 visual organs, must lose its eyes and limbs before it can fulfil the 

 destined purpose of its creation. 



The Epizoa, by which name we recognise the singular class of 

 animals which infest the skin, the eyes, and the gills of fishes and 

 other marine animals, — these external parasites, which are as numerous 

 as, and perhaps more numerous than, the whole class of fishes, — are 



* This name was applied by Linnaus to the genus or group including the para- 

 sitic animals now termed Epizoa, which are classed with the Mollusca in the 

 Systema Natura?, 



