3* CRUSTACEA. 



The separation of the sterna is followed out with more 

 difficulty in the crayfish ; and this, as well as its small size 

 and the strong attachment of the last ring of the thorax 

 to the first ring of the abdomen, makes it a more difficult 

 subject for general study than the lobster. 



Fig. 8, A. /»'. shows the shield and thorax after their 

 separation. The thorax is seen from above. The 

 cavities (Xos. 2-9) are marked by numbers corre- 

 sponding- with those of Fig. 9. The articular cavities 

 of the first pair of maxillipeds cannot be seen in this 

 figure, but are represented in Figs. 9. 9 A, a,/ 1. 



The carapace, thus separated from the thorax, is no 

 longer a cephalothoracic shield, but a cephalic or head 

 shield.*' Having thus artificially separated the con- 

 solidated thoracic rings with their appendages, the 

 scholars will more readily understand that the cephalic 

 shield is really composed of a number of rings repre- 

 sented by two out of the three pairs of jaw-appendages, 

 the two pairs of antennas, and the eye-stalks, five rings 

 in all, bearing their five pairs of appendages. 



Five rings when the eye-segment is counted, and four 

 when this is not regarded as a ring. Naturalists are di- 

 vided in opinion on this subject. The disagreement arises. 

 in my opinion, from the false notions entertained of a 

 crustacean ring. A ring is usually looked upon as a seg- 

 ment which arises independently and prior to the devel 

 opment of the region in which it occurs. According to 

 this view, any portion having a later origin is not entitled 



* Some naturalists, notably Huxley, are of the opinion that 

 the carapace is under all circumstances a cephalothoracic shield, 

 that portion in front of the transverse or cervical suture belong- 

 ing to the head, and the part back of it to the thorax. 



