416 



CHARACTERS OF INVERTEBRATE ANIMALS 



EYE 



Order 3. INTERMEDIATE CRUSTACEA (Leptostraca) 



The Mud- Shrimps, including Nebalia (fig. 257) and allied 

 genera, form a restricted but interesting group of small marine 

 shrimp -like creatures found in all parts of the world. These 

 forms have attracted a good deal of attention, because they are a 

 connecting-link between the higher and lower Crustacea, and 



some zoologists place them in a 

 separate sub -class. Like the 

 higher Crustacea they are com- 

 posed of a definite and limited 

 number of segments, in this case, 

 however, twenty -one instead of 

 twenty, for the abdomen has an 

 extra segment, and the excretory 

 organs include antennary glands. 

 They also agree with stalk -eyed 

 forms as regards their visual organs. But on the other hand all 

 the segments of the thorax are free, though it and part of the 

 abdomen are covered by a bivalve shield which grows back from 

 the head, and the eight pairs of thoracic appendages are leaf-like, 

 as in some of the lower Crustacea. The first four segments of the 

 abdomen bear forked appendages as in certain other of the lower 

 Crustacea (Copepoda), a further agreement with which is found 

 in the fact that the tail also is forked. In addition to antennary 

 glands they possess other excretory structures (shell -glands) re- 

 sembling those of lower forms. On the whole it may be taken as 

 fairly certain that the Leptostraca resemble in many points the 

 ancestral stock from which the various groups of higher Crustacea 

 have diverged. 



Fig. 257. Mud-Shrimp (Nebalia), enlarged 

 Left half of shield cut away. 



Sub-class 2. LOWER CRUSTACEA (Entomostraca) 



This is an exceedingly large and greatly diversified group, 

 including both marine and fresh-water forms, of which the large 

 majority are very small, and which play a very important part 

 in nature as the food of higher animals. There is a very large 

 amount of variation as regards the number of segments, and a 

 similar wide range in the nature of the appendages, which are often 

 flattened and leaf-like, though in other cases they may exhibit 



