no LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



Remove the first maxilla. The small process in front of it is not considered to be 

 an appendage but is a part of the lower lip. 



The heavy mandible is now exposed. It consists of a single triangular 

 piece with strong teeth upon its inner edge; and a small palp, probably the 

 endopod, folded beneath the toothed margin. Spread the mandibles apart so 

 as to see the mouth opening. In front of the mouth opening is the cushion-like 

 upper lip, or labrum. 



Examine the antenna. In the middle of the ventral surface of its basal 

 segment (coxopod) find the renal opening, the opening of the excretory organ. 

 The long many-jointed filament is the endopod; the thin sharp projection near 

 the base of the filament is the exopod. Draw the antenna. 



The antennule or first antenna has a protopod of three joints from which 

 arise two short many-jointed filaments, which are probably exopod and endopod. 



This investigation of the appendages shows that there are at least five seg- 

 ments to the head, eight to the thorax, and six to the abdomen, or nineteen 

 appendage-bearing segments. If the telson is a segment, as seems reasonable, 

 the lobster consists of twenty segments. Some zoologists believe that the eyes 

 also represent a segment and raise the number to twenty-one, but considering 

 that eyes occur on unsegmented animals it seems probable that they are not a 

 pair of appendages homologous to the others. 



/) The respiratory system and the branchial chamber (Hegner, p. 204) : Study 

 the arrangement of the gills, or respiratory organs, in the left branchial chamber, 

 where they have already been exposed. We have noted that one gill is fastened 

 to the epipod of most of the thoracic appendages. Such gills fastened to append- 

 ages are called podobranchiae. Remove the podobranchia and epipod from the 

 third pereiopod, and observe that two more gills are situated beneath it attached 

 to the arthropodial membrane. These are arthrobranchiae. In the lobster there 

 is still a third set of gills, seen by removing the arthrobranchiae. Under the 

 two arthrobranchiae note a gill fastened to the wall of the thorax and hence 

 called a pleurobranchia. Each thoracic segment has therefore typically four 

 gills, but not all of them possess the full number, as the student may readily 

 discover. The crayfish has no pleurobranchiae. 



Cut off one of the gills and examine its structure. It consists of a central 

 axis bearing numerous delicate threadlike filaments. Examine the cut surface 

 of the axis and note the two canals which it contains, one for blood to enter the 

 filaments, the afferent vessel, and one for it to leave the filaments, the efferent 

 vessel. Note the hole left in the thoracic wall where the gill was removed through 

 which the blood vessels pass. 



Remove all the gills from the branchial chamber and note that the segmenta- 

 tion of the thorax is now visible. Examine the region where the extension of the 

 carapace over the branchial chamber was cut off and see that this extension was 



