MUSCULAR AND ALLIED SYSTEMS. 89 



The muscle-fibres of Medusae are derived from the ecto- 

 derm, and each consists of an elongated contractile fibril 

 connected with either a superficial epithelial cell or with 

 one that has migrated into the mesogloea. The fibres are 

 mainly present as a circular layer in the sub-umbrella, and 

 in this region show a very definite transverse striation, 

 connected no doubt with the sharp swimming contractions 

 of the bell. 0. 0. 55. Huntenan. 



0. & R. Hertwig, Der Organismus der Medusen ; Jena, 

 1878, p. 7. 



C. 9. Two specimens of the Sea-Anemone (Actinia mesembryan" 

 themwri). The upper of the two is expanded, the lower 

 contracted. The contraction is due to muscle-fibres situated 

 in the mesoderm, but for the most part, as in Medusge, either 

 connected with or derived from the ectoderm. The fibres 

 are smooth. 0. C. 55 A. 



0. & R. Hertwig, Jena. Zeitschr., Bd. xiv. 1880, p. 42. 



C. 10. Masticatory apparatus (lantern of Aristotle) and neigh- 

 bouring portion of the test of Echinus esculentus. 



The points of the five teeth project through the central 

 opening in the peristomal membrane. Each of the teeth 

 is supported by a " pyramid," composed of two " alveoli," 

 connected at the base by two pieces forming the " trans- 

 verse arch." The pyramids (jaws) are locked together by 

 five falces or braces ; over these are the five slender radii 

 or compasses. The muscles may be divided into four 

 groups, viz. : five pairs of protrusors of the jaws, they arise 

 from the actinostome between the auriculae, and are chiefly 

 inserted into the transverse arch ; five pairs of retractors 

 of the jaws and openers of the mouth, arising from the 

 auriculae and inserted near the tips of the jaws ; five that 

 arise from, and are inserted into, the neighbouring faces of 

 the pyramids, that approximate the teeth ; and five inter- 

 radials, that prevent the downward dragging of the outer 

 furcate ends of the radii when the jaws are protruded, 

 they might slightly tend to protrude the jaws, but this is 

 already provided for by other muscles. All the muscles 



