136 PHYSIOLOGY. 



laminated condition to the otherwise absolutely structureless perisarc. To the same class of 

 products belongs the gelatinous-looking investment which envelopes the acrocyst in Serfuhria 

 pumila, &c. (see p. 50), and in which a distinctly laminated character may frequently be 

 detected. 



4. Coniractility . 



The existence of a fibrillated tissue in the IIydroida has been already mentioned ; and there 

 can be no doubt that this tissue is endowed with contractility, and is tiie proper seat of the more 

 energetic motions performed by these animals. The act by which the hydranth becomes suddenly 

 retracted when touched is evidently due to the contraction of the longitudinal fibres which are 

 developed on the inner surface of the ectoderm; and the rhythmical and exquisitely graceful 

 movements of the mednsre have their seat in the contractile fibres which are developed on the 

 concave surface of the innbrella and in the velum, and are antagonised by the elasticity of the 

 gelatinous substance which constitutes the chief mass of the umbrella. 



It is, indeed, in the motions of the medusae that we find contractility manifesting itself in 

 its highest degree of intensity among theHi'DRoiDA. In these beautiful zooids the contraction of 

 the sub-umbrellar fibres necessarily diminishes the cavity of the umbrella, and by thus expelling 

 in a jet a portion of the water which had filled it in its expanded state, causes the propulsion of the 

 medusa in an opposite direction, or that in which the convexity of the umbrella is turned forward, 

 wdiile the relaxation of the fibres immediately following their contraction permits the elasticity 

 of the umbrella to come into play, so that its cavity instantly resumes its original capacity, and 

 receives within it a fresh supply of water, to be again expelled as a propelling force by the ener- 

 getic contraction of the fibres. The part played here by the velum would seem to consist chiefly 

 in controlling the diameter of the aperture through which the jet of water is propelled from the 

 umbrella cavity. 



But contractility is by no means confined to the fibrillated tissue. There arc many parts of 

 the Hydroida in which no trace of fibres can be detected, and which are yet eminently contrac- 

 tile. Thus the broad four-lobed lip of the medusa of Obelia geniculata is remarkable for its 

 mobility and its power of constantly changing its form while under observation, and yet it 

 consists exclusively of a single layer of differentiated masses of protoplasm (membraneless cells) 

 without the slightest trace of fibres. 



Ecker has shown that the body of Hydra viridis is to a great extent composed of a contrac- 

 tile, semifluid homogeneous substance, agreeing in all essential points with the sarcode matter of 

 the lowest animals. Ecker, however, is wrong in denying a cellular structure to Hydra, for the 

 sarcode is not only differentiated into distinct cell-masses, but these masses are included each — 

 in most cases along with certain granular products — within a proper cell-wall. By the rupture 

 of the cell-wall the protoplasm can be liberated under the microscope ; and Ecker has seen it 

 then undergoing evident contraction, forming isolated masses, which continually change their 

 shape like an Ammha. 



There can be little doubt that many of the motions of the Hydroida are due to the con- 

 tractility of this homogeneous sarcode ; and while we may refer the sudden retraction of the 

 hydranth when touched to the action of the fibrillated tissue, its subsequent slow extension would 



