34 CRUSTACEA. 



thoracic region, while, on the other hand, the rings of 

 the thoracic region have also been crowded forwards, 

 is proved by comparison with lower forms. Thus, in 

 Squilla (Fig. 10), and especially in Lucifer (Fig. n), 

 the forward segments of the thorax are less crowded, 

 and finally in Nebalia (Fig, 12), they are not 

 crowded at all. In this remarkable type is seen the 

 typical distribution of the segments, which are de- 

 scribed in all the books as if equally characteristic of 

 these forms and of the lobster and higher Decapoda. 

 There are distinctly six appendages to the head, their 

 rings consolidated and the shield developed backward, 

 but not yet soldered to the thorax. There are also eight 

 appendages to the thorax with every ring definable 

 in the body, and in the abdomen eight rings, six only 

 having appendages. Embryology shows that Nebalia 

 belongs to the same large group as the lobster and 

 Squilla, and leaves but little room for doubt that the 

 body of the Crustacea is divisible into three regions, 

 — the head, thorax, and abdomen. Also that the two 

 former are concentrated in the Decapoda so as to be 

 almost indistinguishable ; and to such an extent that, in 

 the lobster, in place of counting eight rings in the thorax 

 and six in the head, we have to count nine in the thorax 

 and five in the head. The usual mode of stating this 

 in the books leads the general student to look for pre- 

 cisely the same number of rings in the thorax, head. 

 and abdomen in all forms of Crustacea, whereas they 

 vary in the different forms. 



Five cephalic rings in place of six have become 

 consolidated, and the lateral and dorsal pieces have 

 spread backwards in the lobster, until they have cov- 



