174 THE MICROSCOPIST. 



members and their eyes. The males and females do not 

 resemble each other. 



The order Cirrhipeda consists of the barnacles and their 

 allies. In their early state they resemble the Entomos- 

 traca, are unattached, and have eyes. After a series of 

 metamorphoses they become covered with a bivalve shell, 

 which is thrown off; the animal then attaches itself by 

 its head, which in the barnacle becomes an elongated 

 pedicle, and in Balanus expands into a disk. The first 

 thoracic segment produces the "multivalve" shell, while 

 the other segments evolve the six pairs of cirrhi, which 

 are slender, tendril-like appendages, fringed with ciliated 

 filaments. 



In the order Amphipoda, the Gammarus pulex, or fresh- 

 water shrimp, and the Talitrus saltator, or sandhopper, 

 may be interesting to the microscopist. 



The order Decapoda, to which belong the crab, lobster, 

 shrimp, etc., is of interest, from the structure of the shell 

 and the phenomena of metamorphosis. The shell usually 

 consists of a horny structureless layer exteriorly, an areo- 

 lated stratum, and a laminated tubular substance. The 

 difference between the adult and larval forms in this order 

 is so great that the young crab was formerly considered 

 a distinct genus, Zoea (Plate XYI, Fig. 131). 



For the preservation of specimens of Crustacea, Dr. 

 Carpenter recommends glycerin jelly as the best medium. 



XVII. INSECTS. Many insects may be mounted dry, 

 as opaque objects. They may be arranged in position by 

 the use of hot water or steam. Those which are trans- 

 parent enough may be mounted in balsam, and very deli- 

 cate ones in fluid. To display the external chitinous cov- 

 ering of an entire insect, it may be soaked in strong liquor 

 potassse, and the internal parts squeezed out in a saucer 

 of w^ater by gently rolling over it a camel's-hair brush. 

 It may be put on a slide, and the cover fastened by tying 

 with a thread. It should then be soaked in turpentine 



