38 CRUSTACEA. 



one side can be called the lateral homotype of the 

 same organ on the other side. The lobster's append, 

 ages and rings are also excellent illustrations of what 

 naturalists mean by the use of the word homologous: 

 they mean organs which, in different animals, arc simi- 

 lar in position with relation to each other and in the 

 elements of their structure. Thus the antennae and 

 mouth-parts in Nebalia (Fig. 12), Branchipus (Fig, 

 39), and Lobster (big. 8) are all respectively homol- 

 ogous to one another, or the corresponding abdominal 

 appendages, or the stalks of the eyes, but these last are 

 not homologous with the sessile eyes of the Gam-j 

 marus (Fig. 21) or of the Limulus (Fig. 31). The 

 compound eyes themselves are not in the same rela- 

 tive positions in all Crustacea, so far as we know, and 

 they on this account are considered as homologous 

 organs provisionally, whereas the primitive eyes are un- 

 doubtedly homologous. 



In order to study the elements of a ring, the third ah- 

 dominaJ segment should be separated from the others 

 (Fig. 8, E). Its upper portion, called the tergum (fg). 

 is seen to be convex, and much broader than the almost 

 straight ventral beam or sternum (s/i). The sides of the 

 shell are prolonged downward, forming a double project- 

 ing piece called the pleuron (J>r). In reality, the ring is 

 much more complex in structure, as is show 11 by Fig. S, I), 

 where the tergum, pleuron, and sternum are each seen to 

 be composed of two pieces. Though the sutures marking 

 the limits of all these pieces are not seen in any singk 

 ring, yet there are marks on the outside of the shell which 

 show all the divisions. Thus along the middle of the 

 shield is the mark of the median suture (Fig. 6, sur) 

 of the tergal plates, and on the ventral side, the middle 



