40 CRUSTACEA. 



In Squilla (Fig. 10), of which mention has already 

 been made, the head region has been well defined, and 

 consists of three distinct, movable rings, bearing their 

 appendages, the eye-stalks, and two pairs of antennae. 

 This region is therefore quite independent of the mouth, 

 which lies some distance back of it. The mouth, with 

 its appendages, is followed by the thoracic region, 

 while the latter is only in part concealed by the cara- 

 pace, so that its unconsolidated rings may be readily 

 counted. 



Following the thorax is the abdomen, the most 

 conspicuous region of the body, with its large, well- 

 developed swimmerets, which bespeak for Squilla 

 excellent powers as a swimmer. It is unfortunate that 

 specimens of Squilla cannot be easily obtained, as ii 

 is a much simpler expression of the crustacean plan 

 of structure. In the strange type known as Lucifer 

 (Fig. 1 1 ) , the eye and antenna] segments are prolonged 

 very far in advance of the mouth, and show the ex- 

 treme modifications of this tendency. 



Comparing the lobster with the Squilla and Lucifer 

 type, we find that, in proportion to the size of the 

 two animals, the abdomen of the lobster is much 

 smaller and shorter, and the swimmerets (excepting 

 the last pair) smaller and weaker. The thorax has 

 been carried forward under the carapace] while the 

 walking-legs have become large and strong. The three 

 most essential of the cephalic segments have been car- 

 ried back towards the mouth, and these, together with 

 the mandibular and maxillary segments, are described 

 as the cephalic region, though, more strictly speaking, 

 they might be named the mouth region. The mouth, 



