CRUSTACEA. 3,3 



to the name of ring. Observations upon the eve-segment 

 and eye-stalks of the lobster tend to prove that the former 

 is not' a primitive division of the body, like the other seg- 

 ments, hut that it and the eye-stalks are later develop- 

 ments. Whether, however, the portion to which the eye- 

 stalks are attached grows in the same way as the other 

 rings, or in a different way, it is a fact that in the adult 

 lobster there is a distinct eye-segment bearing the dis- 

 tinctly articulated appendages, the eve-stalks. Further- 

 more.' in Squilla ( Fig. io), a crustacean found on our coast 

 south of Cape Cod. their is a distinct, movable rye-segment. 

 The researches of Professor W. K. Brooks appear to settle 

 this disputed question by showing that <>f the three anterior 

 segments in Squilla the eye-segment arises first, and the 

 two antenna! segments afterwards. These three rings are 

 developed, according to the same authority, from the an- 

 terior end of the consolidated cephalic region. This looks 

 as if they were quite distinct in their mode of origin from 

 the other rings, though, at the same time, they are true 

 segments, each bearing its pair of appendages. The 

 teacher, therefore, may count five rings, and five pairs of 

 appendages belonging to the cephalic region of the lobster. 

 The teacher should always carefully distinguish between 

 the stalk itself and the eye. This association is not com- 

 mon to all the Crustacea, but only to one division, as will 

 be seen in Cammarus and other lower forms. The large 

 compound eyes are normally only secondary developments, 

 and though functionally more powerful and useful, are 

 supplemented by a pair of single eyes, which are found in 

 the young forms, and still exist in some adults, as in the 

 Limulus, or horse-shoe crab (Figs. 31. 32), common on 

 our coast. These are the primitive eyes, though super- 

 ceded and rendered useless in the adults by the functional 

 compound eyes. 



That the rings of the head have become consolidated 

 and extended backwards, covering up the rings of the 



