102 Descriptions of Preparations. 



spends more or less exactly with the curved line already described 

 at page 96 supra, as limiting* off in the omo-stegite an area repre- 

 senting the coalesced terga of the thoracic segments anteriorly 

 from one similarly representing the coalesced terga of the five 

 abdominal segments posteriorly. In having thus its anterior edge 

 or boundary not prolonged beyond the region of the abdomen, the 

 heart of Crustacea corresponds with the heart of all other Arthro- 

 poda, and furnishes an excellent illustration of the value in mor- 

 phology of identity in the relative positions of organs which may 

 differ very widely in external shape and appearance. 



For a figure of the heart of a Decapodous Crustacean, see Professor 

 Owen, Comp. Anat. Invert., p. 318. 



For a discussion as to the venous system, and as to the prolong- 

 ation or non-prolongation of the branchio-cardiac vessels as 

 distinct tubes through the pericardial sinus into the ventricle, 

 see Straus Durckheim, Considerations generales sur TAnatomie 

 Comparee des Animaux Articules, p. 346 ; Milne-Edwards, 

 Histoire Naturelle des Crustace's, p. 103, 1834; Le9ons sur 

 la Physiologic et FAnatomie Comparee, tom. iii., p. 183, 1857, 

 where the views of Straus Durckheim are adopted. 



For the Alae Cordis and their functions, see Verloren, Sur la Circu- 

 lation dans les Insectes Mem. Couronn et Mem. Sav. Etrang. 

 Acad. Roy. Belgique, tom. xix., pp. 68-70, 1844, where the 

 alae cordis are said not to be muscular even in insects. 



34. Common Crayfish {Astacus Fluviatilis), 

 Male, 



Dissected so as to show its stomach, intestine, reproductive and respiratory 

 systems, in situ. 



The greater part of the tergal elements of all the segments of 

 the body have been removed, together with the venous sinuses 

 which underlaid them, as also the heart and its arteries. An 

 arcuated plate, the ' cardiac ossicle,^ is seen crossing the cardiac 

 portion of the stomach, and receiving the insertion of the anterior 

 gastric muscles, which arise from the ventral base of the triangular 



