502 



CRUSTACEA. 



The Astaci are widely separated from the Scyllari and Palinuri, hy 

 Milne Edwards, on the ground of their having a basal scale to the 

 outer antennas, while the others have none. But this distinction is 

 not allowed by him to lead to a subdivision of the Thalassinidea, 

 which it should do, if so important. De Haan makes the same sepa- 

 ration, although he rejects the character derived from the basal scale 

 of the antennae, and rests his distinction on the union of the sides of 

 the carapax to the epistome, which in the Astaci is less perfect, or by 

 a suture. But De Haan, as stated, groups with the Astaci the Tha- 

 lassinidea, a widely divergent group, many species of which show no 

 trace of this union of the carapax to the epistome, any more than Palae- 

 mon and Penaeus. The Astaci form the transition between the other 

 Astacidea and the Caridea ; and it does not seem desirable or proper 

 to make a separate division to include alone these transition species. 



We have already remarked, in the course of our observations on the 

 Anomoura, respecting the propriety of separating the Galatheidea and 

 iEgleidea from the Macroura. Should these groups still be retained 

 among the Macroura, they would form together a section, Anomoural 

 in affiliation, having the posterior thoracic leg, short and inflexed 

 alongside of or beneath the carapax ; the former constituting one family 

 in the section, having the branchiae made up of serial leaflets, and the 

 latter, another family, having the branchiae made up of clusters of 

 filaments. 



There is a singular group of Crustacea, which has been referred 

 to the Macroura, that includes the genus Cuma, and some others allied. 



These animals have many marks of degradation, or rather, of imma- 

 turity. The branchiae are reduced to a single pair ; the eyes are covered 

 by the carapax, and hardly moveable ; the abdominal appendages are 

 generally obsolete, and the caudal pair is styliform ; the carapax is short, 

 leaving three posterior segments of the thorax as distinct segments or 

 rings, very much a.s in Cyclops. If actually mature, they should con- 

 stitute another division of the Macroura, — the Cumidea, Entomostracic in 

 type. But, according to recent observations by Professor Agassiz, 

 communicated by him to the author, the Cumae are in part, if not 

 always, the young or immature forms of certain Macroura, as Alpheus, 

 Palaemon, and Hippolyte. This distinguished observer has actually 

 obtained Cumae from the eggs of Crangon septemspinosus, Palcemon 

 vulgaris, and Hippolytt aculeata. 



