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.^^' 



"With many Crustacea, the internal surface of the skeleton in widely 

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

 manifold form ; some of these serve 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 

 larcfe 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 

 arrano-ement, 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 msscles 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 Savi^ny, Ul-m. &o. part I. and Erich.ion, 3 With the Myriapoda, the transverse muscles 



Entomograpii. iia. 1, 184U, p. 1, Taf. II. pass ofif right and left from the ventral median hue 



1 For the muscles of Astaius, see fVill, in Mül- to the sides of the abdominal segments. With the 

 ler^s Arch. 184:J p. 358.* Lernaeodea, and Ergasilina, there are, under the 



2 In the tail o'f many Decapoda, and in the ab- sldn, transverse as well as longitudinal muscles, 

 domiual segments of Myriapoda. 



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



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

 taeea (^Argulus, Artemia, Branchipus), see Lei/- * [ § 267.] For a circular muscle quite extraor- 



diff, loc. cit. Siebold and KüUi/cer's Zeitsch. II. p. dinary in Callous, 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, PI. 



be easily separated into fibrillae wliich ai)pear to IV. fig. 7, r. — Ed. 

 consist of piles of discs e.xaclly as with some of the 



