S'i THE XATTJRAL HISTOBY EETIEW. 



but think that Dr. Gerstaeeker would have done better had he 

 allowed more weight to those *' many essential differences" which he 

 admits to exist between the forms thus grouped together within the 

 limits of a single order, of which the nature of the respiratory organs 

 is the most striking, especially as these differences are reflected, or 

 at least accompanied, by corresponding peculiarities even in the ex- 

 ternal structure. His order Arthrogastra consists of essentially 

 incongruous elements, and we cannot expect ever to see it generally 

 adopted. Of the remaining orders of Arachnoidea little need be 

 said, — the Araneina and Acarina constitute well-marked groups, as 

 to the limits, of which there can be little difference of opinion, and 

 the only change adopted by Dr. Gerstaeeker consists in the separa- 

 tion from the latter of the Tardigrada and Linguatulina, of which he 

 makes distinct orders. The Pycnogonidae are also justly regarded by 

 Dr. Gerstaeeker as Arachnoidea, — he places them in a distinct order, 

 to which he gives the name of Pantopoda, in allusion to their being 

 apparently composed only of legs. 



In his general arrangement of the Crustacea, Dr. Gerstaeeker 

 seems to us to have been particularly successful, and as this class, 

 from the multitude of forms which it includes, and the marvellously 

 varied life-history of its members, is perhaps the most important and 

 interesting to the experienced Naturalist of all the Arthropod classes, 

 and at the same time the most difficult for the student to obtain a 

 clear notion of, we cannot but regard it as a fortunate circumstance 

 that, in this Manual of Zoology, it has been treated in so philosophical 

 a spirit. 



The orders of Crustacea recognised by Dr. Gerstaeeker are seven 

 in number. In the first and highest of these, under the name of 

 Dccapoda, he includes the whole of the Podophthalmous Crustacea, 

 justly considering that the Stomapoda, whilst still constituting a 

 subordinate group, form an essentially uninterrupted series with the 

 other Decapoda, from which they are distinguished by characters of 

 less than ordinal value. Pollowing the example of Kroyer and Spence 

 Bate, Dr. Gerstaeeker unites the Whale-lice to the Amphipoda, 

 and thus gets rid of the Latreillian order Lremodipoda, — the family 

 Pranizida3, including the single genus Anceus (of which the researches 

 of M. Hesse have shown Praniza to be only a larval and female form), 

 is placed in an appendix to the Isopoda, the author remarking justly 

 upon the singular resemblance of these curious Crustacea to the 

 Decapoda, which, with other peculiarities, renders it somewhat diffi- 

 cult to settle accurately their systematic position. 



