176 



DESCRIPTIONS OF PREPARATIONS. 



The following notes of the differences observable between the Lobster (Ho- 

 marus) and the Crayfish, with which it is sometimes grouped in the family Astacidae, 

 may be useful to the student. 



The squame of the second antenna is relatively small ; the fifth thoracic sternum 

 is fixed ; the first abdominal appendage in the male is two-jointed, and the terminal 

 joint is lamellate but only curved to form a gutter, not rolled up as in Astacus ; the 

 second abdominal appendage in the same sex has a small plate to represent the 

 rolled up lamina of Astacus : in the female the first abdominal somite bears appen- 

 dages well developed ; the telson has no transverse suture. The number of podo- 

 branchiae is as in Astacus, but the stem is completely split into a plume and lamina, 

 and the branchial filaments are stiff "and close set ; the single arthrobranch of the 

 second maxilliped in Astacus is absent, and the total number of these organs is 

 hence reduced to ten, but there are four pleurobranchiae. The infra-oesophageal 

 ganglion is small relatively to the thoracic ganglia, and more distinctly constricted 

 at the sides than in Astacus. The caecum of the mesenteron is small and bilobed : 

 the caeca of the liver are short and the anterior portion of the gland large. The 

 first part of the intestine is smooth, the terminal portion plicated, and at the junction 

 of the two parts there is a dorsal caecum as in Amphipoda and most Decapoda. The 

 testicular lobes are long and only joined by a commissure, and the vasa deferentia 

 are shortened. The largest Gregarine known, Porospora gigantea, inhabits the 

 alimentary canal : see Schneider, A. Z. Expt. iv. 1875 ; E. van Beneden Q. J. M. 

 xii. 1872. 



The Lobster quits the egg in the J/jw-stage, cf. Rathke, A. N. H., 1841 ; 

 Sars, Vidensk. Selsk Forh. Christiana, 1874; Smith, Trans. Connecticut Acad. of 

 Arts and Sciences, 1873. 



The anatomy of a Crab (Callinectes) and its developmental forms or foaea is 

 given in W. K. Brooks's Handbook of Invertebrate Zoology, Boston, 1882, 

 p. 1 68 et seqq. For the Zoaea of the Urachyura, see also Balfour, Comp. Em- 

 bryology, i. p. 398. 



Crustacea. Woodward, Encyclopaedia Britannica (ed. ix.) vi; Gerstaecker, 



