AMPHIPODA. 309 



thoracic limbs. The last pair is, as in all Isopoda, wanting. In a later stage 

 (Cryptoniscus stage) this missing pair of limbs develops. The larvae of this 

 stage become sexually mature as complemental males, protandrous hermaphro- 

 ditism occurring here as in all other Isopoda (Bullar, P. Mayer). At a later 

 stage they change into females or into the final degenerated male forms (Giard 

 and Bonnier, No. 167). 



15. Amphipoda. 



The embryos of the Amphipoda, which, as has been previously 

 mentioned (p. 148), are distinguished from those of the Isopoda 

 by the ventral curvature of the body within the egg, already have 

 the full number of segments and limbs of the adult. Even the 

 fusions which take place between special segments in some forms 

 are already present in the embryo (F. Muller, No. 16). Metamor- 

 phosis is consequently reduced to small alterations of shape, increase 

 in number of the joints and olfactory filaments of the antennae, and 

 additions of setae and teeth. 



A somewhat more pronounced metamorphosis is found in the Hyperidea. 

 In the newly hatched young of Hyperia, F. Muller found no rudiments of 

 pleopoda, whereas Claus, in a Hyperia parasitic on a Discomedusa, found, at 

 a similar stage, the pleopoda and uropoda already developed. As a rule, the 

 young of the Hyperidea, as compared with the adults, in which the eyes are 

 often excessively developed, are remarkable for the smallness of the eyes, and 

 consequently of the head. They are often further distinguished by the shape 

 of the limbs. The young of Phronima, for example, have no powerful pincers 

 on the antepenultimate pair of limbs (Pagenstecher). Spence Bate (No. 2) 

 and more recently Claus (No. 177) have published accounts of remarkable 

 differences between the young and the adults, some of which can be referred 

 to the general manner of life. The larvae of Rhabdosoma thus appear remark- 

 ably compact ; those of Eutyphis, on the contrary, are long. The Rhabdosoma 

 larva recalls in its structure the genus Vibilia. The abdominal limbs first 

 appear in the form of small rudiments. The Eutyphis larvae resemble in 

 appearance the Gammaridae, so that the derivation of the Hyperidea from the 

 Crevettina seems supported by the young stages (Claus, No. 177). 



16. General Considerations Eegarding the Development 

 of the Crustacea. 

 The study of the metamorphosis of a group so varied and so rich 

 in forms as the Crustacea is one of the most attractive and interesting 

 pursuits for the morphologist. Great importance has repeatedly been 

 attached to individual larval stages from a phylogenetic point of view. 

 Although, in recent times, the Natqrtius and Zoaea have been 

 robbed of their glory as racial forms of the Crustacea, the study of 

 Crustacean metamorphosis does not lose all phylogenetic significance, 

 inasmuch as very distinct indications as to the relationships of the 

 different groups are afforded by the manner of their development. 



