58 A Guide to the Zoological Collections in the 



extent as to have lost all resemblance, not only to its 

 Crustacean relatives, but to an animal of any sort, — all its 

 organs, except its genital glands, have degenerated and 

 atrophied, and in its final stage it is, as the exhibited 

 specimen shows, a mere shapeless bag of eggs. 



The sexes in the Cirripedia are commonly united iu 

 the same individual, but in some genera, of which Scal- 

 pellum is one, there are also what are called "comple- 

 mental males " which attach themselves like parasites to 

 the hermaphrodites. One of these small parasitic comple- 

 mental males is seen in one of the exhibited dissections of 

 Scalpellum, 



The Cirripedia are represented in Case 48 by Scalpel- 

 lum and Verruca from the depths of the Andaman Sea and 

 by Lepas and Conchoderma from the German Ocean. 



b. CRUSTACEA MALACOSTRACA. 



[Qrii0£0 37-42, 49-55, hi, 59, 64-69]. 



There are six orders of Malacostracous Crustaceans — 

 Amphipoda, Isopoda, Cumacea, Stomapoda, Schizopoda, 

 and Decapoda — all of which are represented in the exhi- 

 bited collection. 



In the Malacostraca the body consists typically of 2 \ 

 segments, all of which, except the last, bear a pair of jointed 

 limbs. From these 21 segments, however, some zoolo- 

 gists exclude the segment that bears the eyes, and others 

 exclude the terminal limbless segment or telson — thus re- 

 ducing the number to 20, or to 19. 



Of these 21 segments six are always immovably fused 

 together to form a head, the limbs or appendages of which 

 are known in order from before backwards as eye-stalks, 

 antennules, antennae, mandibles, first maxillae, and second 

 maxillae. Of the remaining 15 segments eight form the 

 thorax and are often more or less fused together and to 

 the head to form a cephalothorax, and their limbs or 

 appendages are modified to form foot-jaws and prehensile 

 (often pincer-like), or walking, or rarely swimming, legs : 



