90 INVERTEBRATE ZOOLOGY. 



apoclia. Cut off a single parapodium with the scissors and examine in a 

 watch crystal. It may afterwards be flattened and mounted. The parapo- 

 dium is double, consisting of dorsal and ventral portions. On the dorsal half 

 notice : — A flat triangular piece, the gill — connected with this a slender pro- 

 cess, the cirrus — a fleshy lobe bearing several bristles (setae), and a single me- 

 dian spine {aciculum). Comi)are the ventral half. What part is lacking 'i 



Type XI.— Cambarus sp ?* 



[Astacus or other cray-fish will do as well. Hoiiuirus may be used for class 

 demonstrations. ) 



66. E.rfenial anatomy. Cray-fish may be killed l)y placing them under 

 a bell- jar and adding a bit of cotton soaked with chloroform ; or by immer- 

 sion for a few moments in strong alcohol. In the latter case they should be 

 removed as soon as dead, to prevent the shrinking of the tissues which would 

 occur if the alcohol were allowed to penetrate the interior. The specimens 

 may be examined in the air, but should be dropped into water at least once 

 an hour, that the internal parts may remain moist. A jar containing one or 

 two living specimens should be placed on each table for study of the use 

 of external parts. The parts may be studied in the following order : 



I. General Form. 



Bilateral symmetry, with dorsal, ventral, two lateral, anterior and poste- 

 rior aspects. What other types have been bilateral ? Is the distinc- 

 tion between dorsal and ventral more or less in Lumbricns .^ in As- 

 caris ? Which shows higher development V 



Skeleton. This is an exo-skeleton, consisting of a chitinous cuticula, rein- 

 forced by mineral salts. Test the carapace with a drop of HCl. 

 What does it prove ? Notice the structure of the joints {arthra), del- 

 icate membranes extend between the hard pieces and allow free mo- 

 tion. Notice how the hard pieces overlap when parts are contracted, 

 cf. dorsal pieces of abdomen. Compare the whole with mediaeval 

 armor. 



* The arrangement of Crustacean types is the reverse of the usual order and is rather that 

 of convenience in obtaining material, than that of logical sequence. It is supposed that 

 Type XI will be studied at the opening of the college year (Sept.— Oct.), and is followed by 

 forms that may be kept alive iu laboratory aquaria, and by marine forms that must be 

 studied in alcohol. The natural order, showing the derivation from polychaetous Annelids, 

 would be the following :— (1) Polychaete. (3) Branchipus. (3) Cyclops. (4) Porcellio. (5) 

 Cambams. 



