838 ZOOLOGY. 



of that substance chitin, which we have already met in the 

 gill-bars of Amphioxus. The presence of a cuticle is very 

 jommon in Invertebrate Ccelomates, and contrasts strikingly 

 with the typical condition in Vertebrates. In both cases 

 we have a dead protective layer outside, and a living layer 

 inside, but in the Vertebrates the outer layer is composed 

 of dead cells, and is not a secretion. An epidermis overlain 

 by a cuticle is often called a " hypodermis." The setae are 

 specialized derivatives of the cuticle, each developed from a 

 little sac of the epidermis not unlike that seen in a Mam- 

 malian hair. In the clitellum all the cells of the epidermis 

 are glandular, and the skin is therefore very soft and 

 easily torn. 



Beneath the epidermis come the muscles of the body- 

 wall an outer circular layer, and an inner longitudinal. 

 These layers are of about the same thickness all round 

 there is no such concentration of muscles in the dorsal 

 region as we have been accustomed to in the Vertebrata. 

 Correlated with this is the complete absence of vertebral 

 column or notochord, or indeed of any internal skeleton 

 (endoskeleton) : the only skeleton in fact is the cuticle with 

 its setae (exoskeleton). Another contrast with the Verte- 

 brata is that the central nervous system (nerve-cord) is 

 ventral in position. 



3. The Coelom. If a longitudinal section were taken 

 it would be found that the ccelom, which extends almost 

 from end to end, is divided up into a metameric series of 

 chambers by vertical partitions called septa, partly mem- 

 branous, partly muscular, corresponding to the external 

 divisions between the somites. This partitioning of the 

 ccelom is not complete, however, and there is free com- 

 munication from one end of the body to the other for the 

 coelomic fluid. At the front end of each somite there is a 

 direct opening from the coelom to the exterior, in the 

 middle dorsal line : this opening is called the dorsal pore, 

 and is controlled by muscles. The coelomic fluid contains 

 large amoeboid cells, like blood-corpuscles. 



4. The alimentary canal is a straight tube (fig. 176). 



