I'll YUM MrrilL'ol'itnA <M) 



line Ix'twccn the ^ills of successive scuniriils is kiKtwii as tlic 

 epipod. This is very much reduced in the cruyfish. Tiiru 

 tlie epipods so as to expose the two ar(hri)branrhi(r (joint fjill) 

 which are fixed to the flexible interarticular nienihrane connectinjr 

 the liasal joint of the leg to the thorax. In the lobster sHghtly 

 dorsad of the arthrobranchia' may be found the plfumbrnnrhin 

 (thoracic wall u;ills) which fz;row out from the sides of the thoracic 

 walls. 



Ivxamine the region where th(> gill cover was cut away and ii«iii- 

 that this extension of the carapace over the branchial chamber 

 is not the true sides of the thorax but rather an outward and 

 downward growth from the carapace. The gills conceal the true 

 l)()dy wall. 



Slice off a portion from the tip end of the eye stalk, including 

 only the outer pigmented covering. Remove the pigment from 

 the under surface and mount the transparent piece (cornea) in 

 water on a slide. 



Make a sketch of several corneal facets on the plate of the 

 dorsal aspect of the animal. 



The balancing organs or statocysts are located in the dorsal 

 portion of the basal segment of the antennules. They consist 

 of a pocket or invagination of the surface, which is lined with 

 sense hairs, among which solid particles are lodged. 



Remove the right antennule and cut off the ventral wall of the 

 basal segment. Investigate the saclike statocyst, determining 

 its relations, contents, and the nature of its lining. 



IVIany fine hairlike processes of chitin on the body and append- 

 ages are of a tactile nature, while others on the antennules, 

 antennip, and mouth appendages provide a gustatory service. 



The Appendages. — The 19 pairs of appendages exhibit con- 

 siderable diversity of structure in difTerent segn»ents of the body: 

 yet all possess the same structural plan and in early stages of 

 development are quite similar. Among these appendages there 

 is a close correlation between structure and function, a fact 

 which bears witness to their value as good material for the study 

 of serial homology. 



Typical Segment and Pair of Appendages.— As the abdominal 

 segments are most distinct and less modified than tho.se of other 

 regions, the third or fuinih, with its appemlages, may be studied 

 as a typical somite. Although the regions of the exoskeleton 

 constitute a continuous whole, it is convenient to speak of the 



