268 



LECTURE XIII. 



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

 such grotesque forms. 



These metamorphoses, moreover, lead to very different results, as 

 respects the powers of their subjects, from those of the Medusa and 

 Comatula. The Epizoa and Cirripeds acquire increase of bulk and 

 organs of generation ; but, in every other respect, the varied course of 

 their development ends in a retrograde movement.* Their develop- 

 ment 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 ani- 

 mals, — these external parasites, which are as nu- 

 merous as, and perhaps more numerous than, the 

 whole class of fishes, — are distinguished in their 

 mature state by a body of a more or less elongated 

 or sub-cylindrical form, defended by a smooth, semi- 

 transparent, parchment-like integument, having a 

 more or less distinct head, and generally a pair of 

 long cylindrical ovisacsf {Jigs. 112, 114,/), depen- 

 dent from the opposite extremity of the body. 



In this low organised class of Articulate animals, 

 as in the classes which commence all other great pri- 

 mary groups, there is an extensive gradation of 

 forms by which we pass from species slightly ele- 

 vated above the cavitary Entozoa to the true Crus- 

 taceans. 



The lowest and most simple Epizoa adhere by a 

 suctorious mouth {Jig. 114, a), and traces of ex- 



* LXXXIV. p. 155.(1843.) " ]1 faiit done distinguer 

 avec soiu les formes zoologiques qui peuvent etre assimilees a 

 celles qui produirait \m arret de developpement chez d'autrcs 

 animaux de la meme serie, et cclles qui resultent d'un de- 

 veloppement recurrent." — Milne Edwards, "Aunales des 

 Sciences." Feb. 1844, p. 77. 



f By these appendages the Epizoa are distinguished from 

 those highly-organized Entozoa — the Linguatul(s, e.g., — 

 which by the inferior size of the male, the complex structure 

 of the female, the two pairs of articulated and iincinated limbs of 

 the larva, and the grade of development of the nervous system, 

 seem to make the nearest approach to the present group. 



