CRUSTACEA MALACOSTRACA. 



forms there has been occasion to describe changes in the size of the species according to the localities 

 with a little more detail. 



It is not possible at this place to give the more general results which may be deduced from 

 the following account of the data. When the Malacostraca (and in the end all Crustacea) are com- 

 pletely worked out, it will be possible to give a summary such as that mentioned and to base it 

 securely on references. A few introductory remarks will be given for the separate orders, but on the 

 other hand it has been considered unnecessary to give lists of the literature. 



Lastly, the author wishes it to be distinctly understood that his beginning with the Decapoda 

 does not mean that he considers them higher than the other orders. The arrangement of the orders 

 has nothing to do with this question, as it has been chosen out of regard for considerations which 

 have nothing to do with the systematic arrangement of the Crustacea, namely, such as were imposed 

 by the work itself and which tended only to make a convenient subdivision of the work for the author 

 and for publication. 



I. The Order Decapoda. 



Although the order Decapoda could very naturally in my opinion embrace the Euphausiacea 

 the distance between Euphausia and Sergestes and Peneus not being greater than that between Penens 

 and Homarus or between Homarus and Dromia, yet I have placed the Euphausiacea here as an order 

 by themselves. 



It seems to be an insoluble task to divide this order in a perfectly natural manner into sub- 

 orders. Some authors hold to the old division of Brachyura, Anomura and Macrura (to which the 

 Euphausiacea might thus be added as a fourth suborder); the division is very practical but not of 

 great scientific value. Other authors accept the division made by J. E. V. Boas in 1880 into Reptantia 

 and Natantia. That these two names are in themselves badly chosen is naturally of minor importance 

 if the division otherwise were good. It should be noted, hawever, that the great majority of the 

 Natantia are bottom-animals, some even live in holes in coral blocks or in sponges; it is also inter- 

 esting to notice that the few genera, whose representatives really swim in the upper or deeper water- 

 layers, always so to speak possess peculiar characteristics; thus the two posterior pairs of thoracic 

 legs in Sergestes are modified to true swimmerets while Pasiphae, Hymeuodora and Acanthephyra 

 have retained in use the outer branches of the thoracic legs; lastly, Polybius Henslowi which belongs 

 to the Reptantia and lives a true pelagic life has all four pairs of its walking legs transformed to 

 swimmerets. But, for the rest, I may spare myself the trouble of giving further proof of the bad 

 quality of the two diagnoses, each with ca. 30 characters, which Boas sets up for his two suborders 

 mentioned. In (Termanisering af Dansk Videuskab", Copenhagen, 1895, I have reviewed each of the 

 characters in question one by one and showed that of all the 30 characters there is only one (or per- 

 haps i 1 ;,) which really holds good — and in the same year the quality of the character left was criticised 

 by Th. List. 



In my opinion it will prove impossible to divide the order Decapoda into 2, 3 or 4 suborders 



