CRUSTACEA. 299 



of jaws performed by the basilar joint of the six pairs of thoracic legs, 

 which surround the mouth, the internal branch of those legs forming 

 an ambulatory and prehensile member. The large cephalo-thoracic 

 segment is protected above and laterally by an expanded crescentic 

 shield, obscurely divided by two longitudinal impressions into three 

 lobes, supporting the organs of vision on their highest part. The 

 tergal parts of the segments of the second division of the body are 

 also blended into one trilobate clypeiform piece, their original separa- 

 tion being indicated by the branchial fissures, and the number of the 

 segments by that of the lamelliform appendages attached to their 

 inferior surface. The termination of the intestine beneath the last 

 segment of the second division of the body of the Limulus proves that 

 division to answer to the abdomen in the Malacostraca : but admitting 

 the sessile eyes to indicate a distinct segment, not more than sixteen 

 segments can be determined by the appendages to enter into the 

 composition 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, Daphnia^ and other Entomostraca, to the extent, 

 and in the form, of a bivalve shell, 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 deter- 

 mining the afiSnities of the ancient extinct Crustacea, called Trilobites. 

 These remarkable animals were almost the sole representatives of the 

 present class in the periods which intervened between the deposition 

 of the earliest fossiliferous strata to the end of the coal formation.* 

 Neither antennae nor feet appear to have been found in their remains : 

 the structure of the tergal part only of their bofly-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 six (Ampi/x, 

 Ctyptolitkus), seven (^Ogygid), eleven {Phacops), thirteen i^Caly- 

 mene), to fifteen segments : and the abdomen or tail includes at least 

 eight segments in this CaIymene-\, in which it is bent under the 

 thorax, as in the crab : the abdomen, post-abdomen, or tail, as the 



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



