CRUSTACEA. 



499 



In addition to the remarkable external metamorphosis 

 undergone during the pupa stage, a series of hardly less con- 

 siderable internal changes take place, such as the atrophy of 

 the muscles of the antennae, a change in the position of the 

 stomach, etc. 



Abdominalia. In the Alcippidae the larva leaves the egg as a 

 Nauplius, and this stage is eventually followed by a pupa stage closely 

 resembling that of the Thoracica. There are six pairs of thoracic natatory 

 legs (Darwin, No. 519). Of these only the first and the last three are pre- 

 served in the adult, the first being bent forward in connection with the 

 mouth. The body moreover partially preserves its segmentation, and the 

 mantle does not secrete calcareous valves. 



The very remarkable genus Cryptophialus, the development of which is 

 described by Darwin (No. 519) in his classical memoir, is without a free 

 Nauplius stage. The embryo is at first oval but soon acquires two anterior 

 processes, apparently the first pair of antennas, and a posterior prominence, 

 the abdomen. In a later stage the abdominal prominence disappears, and 

 the antennary processes, within which the true antennae are now visible, are 

 carried more towards the ventral 

 surface. The larva next passes into 

 the free Cypris stage, during which it 

 creeps about the mantle cavity of its 

 parent. It is enveloped in a bivalve 

 shell, and the antennae have the nor- 

 mal cirriped structure. There are no 

 other true appendages, but posteriorly 

 three pairs of bristles are attached to 

 a rudimentary abdomen. Paired com- 

 pound eyes are present. During the 

 succeeding pupa stage the metamor- 

 phosis into the adult form takes place, 

 but this has not been followed out in 

 detail. 



In Kochlorine, a form discovered 

 by Noll (No. 526) and closely related 

 to Cryptophialus, the larvae found 

 within the mantle represent ap- 

 parently two larval stages, similar to 

 two of the larval stages described by 

 Darwin. 



Rhizocephala. The Rhizo- 

 cephala, as might have been antici- 



FIG. 233. STAGES IN THE DEVELOP- 

 MENT OF THE RHIZOCEPHALA. (From 

 Huxley, after Fritz Miiller.) 



A. Nauplius of Sacculina purpurea. 

 B. Cypris stage of Lernaeodiscus por- 

 cellanse. C. Adult of Peltogaster paguri. 



//. ///. IV. Two pairs of antennae 

 and mandibles; cp. carapace; a. anterior 

 end of body; b. generative aperture; c. 

 root-like processes. 



pated from their close relationship to Anelasma squalicola amongst the 

 Thoracica, undergo a development differing much less from the type of the 

 Thoracica than that of Cryptophialus and Kochlorine. 



322 



