OR LIGAMENTS. 123 



the whole body. Broken fragments of the solid skeletons 

 of zoophytes are readily re-united by the exudations from 

 the fleshy substance, of similar ossific matter around the 

 broken extremities, as we see in the testaceous extravascular 

 coverings of higher classes. The delicate vertical, crescentic 

 lamina of the velella is continuous with the more thick 

 horizontal plate on which it rests, and is not detached by 

 maceration. The moveable, lateral, solid plates of the stel- 

 lerida are connected both by irritable and ligamentous bands, 

 as are also the solid jointed moveable spines frequently 

 developed on their upper surface. The plates composing 

 the shells of the echinida are united by harmonic sutures, 

 and the spines are united to the tubercles by the enarthroid 

 form of articulation, where there are marginal muscles, an 

 exterior capsule, and generally a round central ligament, 

 like the ligamentum teres of the femur, as in the cidaris and 

 spatangus. The unconsolidated portions of the general 

 cuticular exudation form the means of connection between 

 the dense external parts of the trunk and its appendices in 

 the helminthoid and in the entomoid classes of diplo-neura. 

 This we already find in the segments of the trunk and of 

 the rudimentary antennse of some of the higher epizoa, in 

 the tough epidermic membrane connecting the pieces of 

 pedunculated cirrhopodous shells, and in the segments and 

 cirrhi of the same animals, and of many of the higher an- 

 nelides. In the more solid skeletons of the entomoid 

 classes, the calcified coverings of the segments of the trunk 

 and of the antennse and palpi are partially retracted within 

 each other, and are connected by numerous muscles, and by 

 the true skin, and its continuous thin epidermis. The other 

 appendices developed from the sides of the body present 

 chiefly the ginglimoid forms of articulation without the aid 

 of ligaments, where one portion of the skeleton is locked 

 into another, and where the motions of the joints are very 

 secure, but limited in extent ; these forms of the articula- 

 tions w r e see especially in the strong members of the larger 

 Crustacea and coleopterous insects. The valves of con- 

 chifera are connected together both by the teeth of the 

 hinge, which are often locked into each other, as in spondy- 

 lus, and by the tough extra-vascular ligament, which gro\vs 

 by successive layers of epidermic matter, and constantly 



