284 CLASS vni. 



may be looked on as the rudiments of a neural skeleton. There is 

 in each division of the thorax a process, which often has the form of 

 the letter Y, supports the nervous cord, and by the expansion of 

 its two branches, which are directed upwards, partly covers it. 

 To this process AUDOUIN has given the name of Entothorax ; it is 

 even found in the head and sometimes in the first abdominal ring. 

 These are the same processes which TREVIRANUS had already com- 

 pared to vertebrae 1 . These vertebrae, however, are not joined 

 together to form a spine, but are separated from each other by 

 certain spaces. The dermal skeleton of Insects consists of a pecu- 

 liar substance to which ODIER gave the name of Chitine, LASSAIGNE 

 that of Entomolme 2 , which occurs also in the integument of Arach- 

 no'idea and Crustacea, and which is not soluble in caustic potas 

 neither is rendered yellow by nitric acid, like corneous tissue. Il 

 burns without fusion or intumescence. It forms different layers 

 which the most external is composed of irregular cells 3 . 



The arrangement of the muscles is different in the different 

 orders of Insects, nay, in the same insect in its different states, if it 

 undergoes complete metamorphosis. The difference between the 

 muscles of the thorax and of the abdomen, which in the per- 

 fect insect is so marked, is absent in the vermiform elongate* 

 larva, for instance, in caterpillars. Along the dorsal and venti 

 surface riband-like muscles run longitudinally ; there are different 

 oblique muscles in addition. The muscles present in their bundle 

 transverse stripes, as in vertebrate animals 4 . They are usuall; 



skeleton ; see especially his excellent work Von den Ur-Theilen des Knochcn- und Scha- 

 len-Geriistes, Leipzig, 1826, folio. 



1 Verm. Schriften, IV. s. 229, 230. 



2 See ODIER, Mem. de la Soc. d'Hist. Nat. de Paris, I. 1823, pp. 29 42, and the 

 later investigations of C. SCHMIDT, Zur vergleichenden Physiologie der Thiere, Braun- 

 schweig, 8vo. s. 32, 52. 



3 Comp. H. FREY and R LEUCKART in the new edition of E. WAGNER, Lehrb. der 

 Zootomie, revised by them, 1845, pp. 3 5 ; also H. MAYER in MUELLER'S Archiv, 

 1842, s. 12 16. In the skin of the Silkworms and their pupae (and also in other pupce 

 of lepidoptera) there are found stellate cells, which PLATTNER compares with the bone- 

 corpuscles in the osseous tissues of vertebrates, MUELLER'S Archiv, 1844, s. 46, 47. 



4 Since in every ring of the Larva's body the same arrangement of the muscles is 

 observable, the number of the muscles, when those of all the rings are counted together, 

 is very great. LYONNET found in the larva of the Willow-hawk more than 4,000 

 muscles. 



