228 MANUAL OF ZOOLOGY. 



mouth, gullet, and intestine, terminating posteriorly in a distinct 

 anus. The nervous system consists of a double ventral cord. 



The embryo (fig. 84, a} is free-swimming, and is provided 

 with visual organs and locomotive appendages. The two 

 sexes are now alike, and the conversion of the active embryo, 

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

 as an instance of " retrograde metamorphosis." In Achtheres 

 percarum (fig. 84), the primitive form of the young is a " Naup- 

 lius ;"* 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 rupture of the same. 



ORDER II. RHIZOCEPHALA. The name Rhizocephala has 

 been proposed for another group of Crustacea, which are fixed, 

 parasitic, and greatly deformed when adult ; but which are lo- 

 comotive when young, and which are most nearly allied to the 

 Cirripedia. The larvae are " Naupliiform," with an ovate unseg- 

 mented body, an unpaired median eye, and a dorsal shield or 

 carapace. The abdomen terminates in a movable caudal fork, 

 and there is neither mouth nor alimentary canal. In their 

 second stage (as so-called " pupae "), the young of the Rhizo- 

 cephala are enclosed in a bivalve shell, the foremost pair of 

 limbs constitute peculiar organs of adhesion (" prehensile an- 

 tennae " of Darwin), the two following pairs of limbs are cast off, 

 and six pairs of powerful biramose natatory feet are formed on 

 the abdomen. There is still no mouth. The "pupae" now 

 attach themselves to the abdomen of Crabs, Porcellance, and 

 Hermit-crabs ; they remain astomatous ; " they lose all their 

 limbs completely, and appear as sausage-like, sack-shaped, or 

 discoidal excrescences of their host, filled with ova ; from the 

 point of attachment closed tubes, ramified like roots, sink into 

 the interior of the host, twisting round its intestine, or becom- 

 ing diffused amongst the sac-like tubes of its liver. The only 

 manifestations of life which persist in these non plus ultras in 

 the series of retrogressively metamorphosed Crustacea are 

 powerful contractions of the roots, and an alternate expansion 

 and contraction of the body, in consequence of which water 

 flows into the brood-cavity, and is again expelled through a 

 wide orifice " (Fritz Miiller). The branched roots of the 

 Rhizocephala appear to be the homologues of the " cement- 

 ducts" of the Cirripedia, and to be, therefore, really the 

 antennae. 



* 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. 



