Description of a Species of Caligus. 241 



the result of minute folds in the muscular fibres ; but we have 

 been unable to detect an approximation of these striae, or any al- 

 teration in their appearance, during the contraction of the muscle : 

 this, however, may be owing to their extreme minuteness.* 



On account of the very peculiar forms and motions of some of 

 the organs in this animal, it contains several muscles of very unu- 

 sual character. We reserve the description of them till these or- 

 gans come under consideration. 



a. Muscles of the Segments of the Body. 



The first cephalic segment is flexed by two short slender mus- 

 cles on each side, (R, R',) situated just exterior to the process which 

 forms the articulation of this segment, (figs. 1 and 7.) and di- 

 rected backward and outward. They unite in a common short 

 tendon. They act in depressing this segment, and assist in at- 

 taching its cup 'and anterior margin. This margin is provided 

 with a narrow ridge, which is striated or wrinkled transversely, 

 like the cup, and is apparently intended to produce a closer at- 

 tachment of this margin. 



For the mutual motions of the cephalic and thoracic segments, 

 there are three pairs of muscles situated in the former, two at- 

 tached near the median line, and one pair laterally. A pair of 

 short muscles (I, I, fig. 7) run nearly parallel with the median line ; 

 they produce the slight flexion admitted at this articulation. An- 

 other pair of muscles, long and large, (S, fig. 7,) are situated 

 on each side of the preceding ; they pass obliquely outward. In 

 addition to aiding in flexion, they produce a lateral sliding motion, 

 often observed between these segments. A third pair (K, fig. 7,) 

 also assist in flexion. The large muscles, (K^,) situated in the 

 posterior segment, appear also to pertain to this joint ; but we are 

 not fully assured that this is really their insertion. 



The extensor muscles of the posterior thoracic segment, and 

 of the abdomen, arise adjacent to the median line, near the cen- 

 tre of the anterior thoracic segment. Three pairs of muscles are 

 attached at this point. The outer (L) pass obliquely outward and 



* Since writing the above, we have found that these striations have been obser- 

 ved by Dr. Hodgkin in the muscles of man. He sa)'s : " Innumerable very mi- 

 nute but clear and fine parallel lines, or striae, maybe distinctly perceived, trans- 

 versely marking the fibrillse." These observations have led Dr. Hodgkin to doubt 

 the globular constitution of the contractile fibre. We have also observed them 

 distinctly in some of the Arachnides. 



Vol. XXXIV.— No. 2. 31 



