320 THE ANATOMY OF INVERTEBRATED ANIMALS. 



growths from the neural aspect of each somite, and, whatever 

 their ultimate form, they are, at first, simple bud-like pro- 

 cesses. Very generally, a broad median prolongation of the 

 sternum of the somite which lies in front of the mouth gives 

 rise to a lahruin y while a corresponding, but often bifid me- 

 dian elevation, behind the mouth, becomes a metastonia. 



In many Arthropods, the haimal or tergal face of the body 

 grows out into lateral processes, which may either be fixed, 

 or more or less movable. The lateral prolongations of the 

 carapace in the Crustacea and the wings of Insecta are 

 structures of this order. 



In a number of Insects belonging to different orders of 

 the class, an amnionic investment is developed from the 

 extra-neural part of the blastoderm by a method similar to 

 that which gives rise to the amnion in the higher Vertebrata, 



In all the higher Arthropods, a certain number of the 

 somites which constitute the anterior end of the body coa- 

 lesce and form a head, distinct from the rest of the body ; 

 and the appendages belonging to these confluent somites un- 

 dergo remarkable modifications, w^hereby they are converted 

 into organs of the higher senses and into jaws. In many 

 cases, the somites of the middle and posterior parts of the 

 body become similarly differentiated into groups of poly- 

 somitic segments, which then receive the name of thorax and 

 abdomen. The somites entering into each of these groups 

 may remain distinct or may coalesce. The tergal expansions 

 of the somites of the head, or of both head and thorax, may 

 take the shape of a broad shield, or carapace. This may con- 

 stitute a continuous whole (e. g., Apus, Astacns) ; or its two 

 halves may be movablv connected by a median hinge, like a 

 bivalve shell ( Cypris, Lhnnadia) ; or, finally, the tergal pro- 

 cesses of each side may remain distinct from one another and 

 freely movable on their respective somites (wings of In- 

 sects). 



Limbs, or appendages capable of effecting locomotion, are 

 always attached either to the head or to the thorax,' or to 

 both. They may be present or absent in the abdominal re° 

 gion. In adult Arachnida and Insecta., there are no abdomi- 

 nal limbs, unless the accessory organs of o^eneration, the stings 

 of some insects, and the peculiar appendages of the abdomen 

 in the Thysanura and Collemhola, be such. 



The alimentary apparatus presents very wide diversities 



» The extinct Trilohites possibly form an exception to this rule. 



