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- 
Jel, 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 mascles 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 Savigny, Mém. &c. part I. and Erichson, 
Entomograph. Hft. 1, 1840, p. 1, Taf. IT. 
1 For the muscles of Astacus, see Will, in Mil- 
Zer’s Arch. 1843, p. 358.* 
2 In the tail of many Decapoda, and in the ab- 
dominal of iapod 
4 'P 
* [§ 267, note 1.] For researches on the inti- 
mate composition of the muscular tissue with Crus- 
tacea (Argulus, Artemia, Branchipus), see Ley- 
dig, loc. cit. Siebold and Kolliker’s Zeitsch. II. p. 
327, III. p. 801). The fibres of muscles can often 
be easily separated into fibrillae which appear to 
consist of piles of discs exactly as with some of the 
8 With the Myriapoda, the transverse muscles 
pass off right and left from the ventral median line 
to the sides of the abdominal segments. With the 
Lernaeodea, and Ergasilina, there are, under the 
skin, transverse as well as longitudinal muscles, 
other Arthropoda, and especially the Insecta, under 
which, this point will be specially treated. — Ep. 
* [§ 267.] For a circular muscle quite extraor- 
dinary in Caligus, see Dana, Descript. of a species 
of Caligus, Amer. Jour. of Sc. XXXIV. p. 247, Pl. 
IV. fig. 7, r. — Ep. 
