ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 51 



ration of its surface. The relation that it holds to the internal viscera is to 

 afford protection and means of siipport. 



When the former only is required, the structure is generally smooth and 

 even ; where the tissues are internally thicker and irregular, it gives to the 

 external surface an indented and irregular aspect, which is common, parti- 

 cularly in the flat and short-tailed Crustacea, where the markings are so per- 

 sistent as to afford a very valuable assistance for the determination of species. 



These markings are generally induced by the attachments of the tissues 

 that secure certain viscera in theii" positions ; these form generally points of 

 depression ; but where any organ (such as the liver, stomach, or branchial 

 appendages) is protected, the corresponding points in the carapace are ele- 

 vations, sometimes crowned with a pointed spine or process. The branchial 

 appendages are external in relation to the body of the animal, but covered 

 over and protected by the lateral walls of the carapace. To complete this 

 so as effectually to protect those organs without pressing on or interfering with 

 their functions, a very considerable amount of lateral development has taken 

 place, and a peculiar reflection so as to bring the margin of the carapace 

 below the branchial appendages and to protect them from rude contact with 

 the limbs. The angle which is induced by this inflection of the carapace 

 over the hepatic lobes and enclosing the branchias is generally well defined 

 and ornamented with points or processes more or less numerous. These 

 processes define the dorsal limits of the carapace. 



Desmarest, half a century since, mapped out the dorsal surface of the 

 carapace into regions coinciding with the limits of the internal viscera. 



Milne-Edwards, in his 'Histoire des Crustaces,' published in 1839, adopted 

 the same views, supporting it by illustrations from several genera. 



Professor Dana more recently, in his great work on Crustacea, has divided 

 the dorsal siirface into many more regions, taking the numerous areolites that 

 are present in some genera (as Zozynms). 



He divides the carapace by a transverse line that extends from just ante- 

 rior to the last of the normal lateral teeth to the same on the opposite side, 

 and separates it into anterior and posterior portions. 



The anterior he again divides into three parts, defined by lines of depres- 

 sion, and names them the median region and two antero-lateral regions. 



The median region covers the stomach, and includes the gastric and genital 

 regions of Desmarest. 



The space anterior to the median region he calls the frontal, and on either 

 side the orbits form another, which may be called the orbital region. 



The posterior portion of the carapace he likewise divides into a posterior 

 and two postero-lateral regions. 



Professor Milne-Edwards in 1854 readdressed himself to this subject and 

 further elaborated it. In the ' Annales des Sciences NatureUes' ho communi- 

 cated his researches with illustrations from several genera, and divided the 

 dorsal surface of the carapace into regions corresponding with the names of 

 the internal viscera. I3ut it appears to me that the correspondence in many 

 parts exists in the name only ; as, for instance, in the gastric region, which he 

 subdivides into epigastric or anterior lobes of the gastric region, protogastric 

 or latero-autcrior lobes, mesogastric or median lobe, metagastric or latero- 

 posterior lobes, and urogastric or medio-posterior lobe of the gastric region. 



It is quite Avithin the power of demonstration to prove that it is more in 

 accordance with the correct anatomical details of the animal's structure if 

 the lobes that he named metagastric, or latere -posterior lobes, were called, 

 according to Desmarest, the genital regions after the viscera they protect. 

 And no advantage appears to me to be derived from dividing a region 



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