312 THE CRUSTACEA. <§) 267 



abortive and fused several together, — serve, in descriptive zoology, to char- 

 acterize orders, sub-orders, families, and genera ; consequently they need not 

 be mentioned here/^' 



AVith many Crustacea, the internal surface of the skeleton in widely 

 different parts of the body, has prolongations and processes of the most 

 manifold Ibrm ; some of these servo as points for the insertion of muscles 

 and tendons, and others as partitions separating and shielding particular 

 organs. 



CHAPTER II. 



MUSCULAR SYSTEM AND ORGANS OF LOCOMOTION. 



§ 267. 



The voluntary muscles of Crustacea are composed exclusively of trans- 

 versely-striated fibres, and are, moreover, perfectly colorless. ''> 



They are always inserted upon the interior of the skeleton, either 

 directly, or by means of its prolongations. These last are often very long, 

 resembling tendons ; from which, however, they differ in their intimate 

 structure and chemical composition. They are composed of straight, paral- 

 lel, flattened fibres, and show their direct relations with the cutaneous 

 skeleton by containing chitine. 



The isolated muscles have usually a riband-like form,* and are especially 

 accumulated in those regions of the body displaying great power or 

 extensive movements. There are, therefore, for their reception, cavities or 

 large canals in certain parts of the skeleton. Generally, the flexors are 

 upon the ventral, and the extensors on the dorsal surface of the body. 

 The first are always larger and more powerful than the second. Usually, 

 the muscles pass from one segment to the adjacent one, and by this 

 arrangement, the interarticular movement between the segments is pro- 

 duced. Their course is longitudinal, but, especially where there are 

 several superposed layers, they assume also an oblique and crucial direc- 

 tion. ^^^ Rarely are transverse m-Ascles observed.''* 



The muscular system in general is very unequally developed in the 

 various orders of Crustacea. It is most complicated when the number of 



1 See Savisny, Mem. &c. part I. and Erichson, 3 With the Myriapoda, the transverse muscles 



Entomograph. Hft. 1, 1840, p. 1, Taf. II. pass off right and left from the ventral median line 



1 For the muscles of Astacus, see fViU, in Miil- to the sides of the abdominal segments. With tha 

 /er's Arch. 1843, p. 358.* Lernaeodea, and Ergasilina, there are, under the 



2 In the tail of many Decapoda, and in the ab- skin, transverse as well as longitudinal muscles, 

 dominal segments of Myriapoda. 



* [§267, note 1.] For researches on the inti- other Arthropoda, and especially the Insccta, under 



mate composition of the muscular tissue witli Crus- which, this point will be specially treated. — Ed. 

 tacea {Argulus, ArteTnia, Branchipus), see Ley- * [ § 267.] For a circular muscle quite extraor- 



dig, loc. cit. Siebold and KoU-'ker's Zeitsch. II. p. dinary in Caligus, see Dana, Descript. of a species 



327, III. p. 301). The fibres of muscles can often of Caligus, Amer. Jour, of Sc. XXXIV. p. 247, V\. 



bo easily separated into fibrillae which appear to IV. fig. 7, r. — Ed. 

 consist of piles of discs exactly as with some of the 



