CHARACTERS OF INVERTEBRATE ANIMALS 



303 



various purposes. The most obvious are those which end in 

 the large pincers, behind which four large pairs of walking -legs 

 are apparent. Under the head are a number of overlapping 

 limbs, turned somewhat forwards, which guard the mouth and 

 act as jaws. In a Vertebrate the jaws are part of the bony 

 framework of the head, helping to bound the mouth-cavity, and 

 the lower jaw works up and down. But the limb-jaws of the 

 Lobster are outside the opening of the mouth, and from the nature 

 of the case work against one another from side to side. To realize 

 this, raise your hands to your mouth and "clap" them together, 

 which will give an idea of the way in which one pair of the 

 Lobster's jaws are worked. Six pairs, however, are present 

 In all. 



By combining the knowledge obtained by dissecting one 

 Lobster from the side and making a cross-section through another, 

 the following distinctive characters of higher Invertebrates can be 

 easily verified: 



1. There is a protective external skeleton (exoskeleton) but 

 no internal skeleton (endoskeleton), i.e. nothing can be discovered 

 equivalent to the skull, backbone, &c., of, say, a Perch, or to the 

 notochord of a Lancelet. The absence of endoskeleton modifies 

 the structure in many ways, as, e.g., in regard to the attachment 

 of muscles. In such a limb as the human arm the numerous 

 muscles are attached to the bones, but in a Lobster's leg they 

 are attached to the firm 



exo-skeleton. 



2. The body is not a 

 double but a single tube in 

 structure. 



3. The side-walls of the 

 digestive tube are not per- 

 forated by gill-slits. 



4. The heart is situated 

 dor sally, the exact opposite 

 of the Vertebrate condition. 



5. The nervous system consists of a ring surrounding trie 

 gullet, and continued backwards into a ventral nerve-cord. The 

 dorsal side of the ring is thickened into a double brain or cerebral 

 ganglion. A very large number of Invertebrates possess such 

 a nerve-ring and ventral cord, while many more have the ring 



LOW NERVE TUBE 

 NEURAL TUBE 

 NOTOCHORD 



B 



NERVE-CORD 



Fig. 176. Diagrammatic Cross-sections through, A, a Verte- 

 brate; B, a higher Invertebrate 



