CRUSTACEA. 165 



answer to the abdomen in the iMalacostraca : but, admitting the ses- 

 sile eyes to indicate a distinct segment, not more than sixteen seg- 

 ments can be determined by the appendages to enter into the com- 

 position of the entire crust of the Limulus, including the sword- 

 shaped appendage, which is analogous to the last or post-anal segment 

 of the higher Crustacea, and consists of a single modified segment. 



In the small Entomostraca, the number of the thoracic and ab- 

 dominal segments generally exceeds that in the Malacostraca. The 

 Branchipus stagnalis, for example, has eleven thoracic segments, and 

 nine abdominal or caudal rings, besides a distinct head protected by 

 a cephalic shield. In the Isaura, in which this shield is developed, 

 as in the Cypris, DapJmia, and other Entomostraca, to the extent, 

 and in the form, of a bivalve shellj enveloping the whole body, the 

 number of thoracic and abdominal segments exceeds twenty -four. 



The distinction between the Entomostraca and Malacostraca in the 

 number of the segments of the body is of the first importance in de- 

 termining the affinities of the ancient extinct Crustacea, called Tri- 

 lobites. These remarkable animals were almost the sole represen- 

 tatives of the present class in the periods which intervened between 

 the deposition of the earliest fossiliferous strata to the end of the coal 

 formation.* They appear to have been without antennae and feet; the 

 structure of the tergal part only of their body-segments is yet known ; 

 but these are grouped together to form a distinct head, thorax, and 

 abdomen or tail. The head is formed by a large semicircular or 

 crescent-shaped shield ; the thorax consists of from ten to fifteen seg- 

 ments ; and the abdomen or tail includes at least eight segments in this 

 Ccdymene f , in which it is bent under the thorax, as in the crab : the 

 abdomen, post-abdomen or tail, as the third segment is variously 

 termed, contains fifteen fettered segments in the Asaphus caudatus : 

 the segments of both thorax and abdomen are very similar to each 

 other, and gradually decrease in size. They are divided by two lon- 

 gitudinal furrows into three lobes. The head supports a pair of large 

 compound eyes situated near the sides, like the large outer pair of 

 eyes in the Limidus, which they resemble in form and structure. 



The Malacostraca are divided into two groups, according to the 

 attachment of the eyes : those with immoveable sessile eyes form the 

 Edriophthalmay those with moveable pedunculated eyes the Podoph- 

 tlialma. 



The lower organised or edriophthalmous forms of malacostracous 

 Crustacea resemble the Trilobites in the non-confluence and uniformity 



* Buckland, Bridgwater Treatise, i. p, 390. f Prep. No. 208. 



M 3 



