SEGMENTATION 5 



of the other inteinal organs.^ In this respect the Crustacea agree 

 with all the other Arthropods, in the adults of which the 

 segmentation is confined to the exterior and to the nervous 

 system, and does not extend to the body-cavity and its contained 

 organs ; and for the same reason they differ essentially from all 

 other metamerically segmented animals, e.g. Annelids, in whicli 

 the segmentation not only affects the exterior and the nervous 

 system, but especially applies to the body-cavity, the musculature, 

 the renal, and often the generative organs. The Crustacea also 

 resemble the other Arthropoda in the fact that the body-cavity 

 contains blood, and is therefore a " haemocoel," while in the 

 Anmelids and Vertebrates the segmented body-cavity is distinct 

 from the vascular system, and constitutes a true " coelom." 

 To this important distinction, and to its especial application to 

 the Crustacea, we will return, but first we may consider more 

 narrowly the segmentation of the Crustacea and its main types 

 of variation within the group. In order to determine the 

 number of segments which compose any particular Crustacean 

 we have clearly two criteria : first, the rings or somites of which 

 the l)ody is composed, and to each of which a pair of 

 limbs must be originally ascribed ; and, second, the nervous 

 ganglia. 



Ai-ound and behind the region of the mouth there is very 

 little difficulty in determining the segments of the body, if we 

 allow embryology to assist anatomy, but in front of the mouth 

 the matter is not so easy. 



In the Crustacea the moot point is whether we consider the 

 paired eyes and first pair of antennae as true appendages belong- 

 ing to two true segments, or whether they are structures sui 

 qeneris, not homologous to the other limbs. With regard to the 

 first antennae we are probably safe in assigning them to a true 

 body-segment, since in some of the Entomostraca, e.g. A'pus, 

 the nerves which supply them spring, not from the brain as in 

 more highly specialised forms, but from the commissures which 

 pass round the oesophagus to connect the dorsally lying brain 

 to the ventral nerve-cord. The paired eyes are always inner- 

 vated from the brain, but the brain, or at least part of it, is very 



^ Tlie mu.scles are to a certain extent segmented in covresiiondence witli the 

 limbs ; and the heart, in Phyllopoda and Stoniatopoda, may liave segmentally 



arranged ostia. 



