ANNULOSA: CRUSTACEA. 285 



or larva, into the swollen and deformed adult, must be regarded 

 as an instance of " retrograde metamorphosis." In Achtheres 

 percarum (fig. 140), as generally in the order, the primitive form 

 of the young is a " Nauplius ; " * but a wholly different larva, 

 resembling the Cyclops 

 in shape, but with fewer 

 limbs and somites, is 

 prepared within the 

 Nauplius-skin, and is 

 liberated by the rup- 

 ture of the same. 



As regards their af- 

 finities, the Ichthyoph- 

 thira are closely allied 



tO the Copepoda, and Fig. 140. Ichthyophthira. a Free-swimming larva of 

 rno^r \r\r\ e.(*r\ H^ v^rror-rl Achtheres percarum in its first stage ; b Adult male 

 may, indeed, DC regard- O f the same. Enlarged. (After Owen.) 



ed as parasitic Cope- 

 pods, having the mouth modified so as to form a suctorial 

 tube or beak, resulting from the elongation of the labrum 

 and labium. Within this are two stylets or lancet-shaped 

 mandibles, used in piercing. The feet are often deformed by 

 age, or wanting, but are primitively natatory. Not only does 

 their developmental history bear out this view, but cases are 

 known (in some Lern&cp) in which the males do not undergo 

 retrograde metamorphosis, but remain permanently in the con- 

 dition of free Copepods. 



ORDER II. RHIZOCEPHALA. Adult parasitic, attached by 

 ramified roots (antenna?). Body sac- like, unarticulated, with- 

 out limbs. No mouth. Larva a locomotive " nauplius." 



The Rhizocephala constitute a peculiar group of Crustaceans, 

 the adults of which are found attached parasitically to the 

 abdomen of Crabs and Hermit-crabs. The body (fig. 141, B) 

 is sac-like, and non-segmented, and consists of a muscular 

 mantle in which no skeletal structures are developed, its only 

 aperture being reproductive and closed by a sphincter. There 

 are no limbs, sense-organs, or alimentary canal, but there are 

 well-developed reproductive organs, each individual, according 

 to Giard, being hermaphrodite. The sac-like body is kept in 

 connection with its host by means of branched, root-like pro- 

 cesses of attachment (fig. 141, B), which sink deeply into the 

 tissues of the latter. These processes appear to correspond 



* The name of "Nauplius" was given by O. F. Miiller to the unseg- 

 mented ovate larva of the lower Crustacea, with a median frontal eye, but 

 without a true carapace ; and this name may be conveniently employed to 

 designate all the larval forms which agree in these characters. 



