CONSIDERATIONS. 469 



In the lobster {Astacus marinus) the proportion of car- 

 bonate of lime is less, and of animal matter and water greater. 

 The proportions of the other component parts do not differ much. 



In the more perfect Vertehrata, the skin, being merely 

 intended to protect the parts beneath from the immediate 

 contact of external objects, presents merely a layer uniformly 

 spread over the whole body ; but, in the articulated animals, 

 the teguments, having to perform the functions of the skeleton, 

 exhibit many internal prolongations and articulations neces- 

 sary for the new uses to which they are destined. In the 

 fishes, the skin sends off internal prolongations at different 

 distances, which serve for the attachment of the muscles of 

 the trunk ; these, in the genus Ammoccetes, corresponding to 

 faint transverse folds of the skin, cause the body to appear 

 articulated. The Annelida ahranchia {Cryptobranchia) 

 offer the same appearance ; but in the other Annelida, the 

 folds of the teguments are more distinct, and we find appen- 

 dages serving as locomotive organs, furnished with muscles, 

 and articulated. The Myriapoda have the teguments solid, 

 the feet fully developed, furnished with numerous muscles, and 

 internally the teguments are produced into many apophyses to 

 which these muscles are attached. 



Lastly, in Insecta, Crustacea and Arachnida, the teguments 

 acquire all the development of which they are susceptible, 

 and become as complicated as the skeleton of Vertehrata. 



The Annelida abranchia have the body composed of a 

 greater or less number of segments, nearly similar, and without 

 any distinct head; but in the others {Gyninobranchia) the 

 first segment becomes a true head, bearing eyes more or less 

 distinct, and tentacula. In some species the head is composed 

 of many segments united, the first prelude of its more perfect 

 organization in the following groups. The body of some 

 Annelida is divided into two parts, one bearing cirrhi, which 

 might be considered as the trunk, the other without cirrhi, 

 representing the abdomen ; but this distinction is very uncer- 

 tain, as in some it is the posterior, in others the anterior 

 segments which bear the cirrhi. In the Myriapoda the body 

 is composed of segments uniformly or alternately similar ; the 

 head is composed of several segments united. Insects have 

 the body divided into eleven, twelve, or thirteen'' segments, 



*• Evidently wrong; the true number being always thirteen, including the 

 head.— D. 



