of the Contractile Substance of the Polythalamia. 353 
bited in the simplest manner by greater or less constriction of 
the ellipsoidal body of the Polythalamia, which occurs slowly and 
slowly changes position. At the constricted spot the contractile 
substance is thickened, and its optic section exhibits the form of 
a narrow sickle with the concavity placed outwards. These 
constrictions are regularly accompanied by passive movements 
of the central substance of the body, which contains the 
vesicles. 
b. On every part of the contractile cortical layer the contrac- 
tile action gives rise to processes in the form of tubercles, warts, 
papillee, also flat knob-like prominences and lamellz, and, lastly, 
elongated, either regular or somewhat irregular processes. These 
prominences and processes are only formed, as far as the present 
observations extend, upon the outer surface of the contractile 
cortical layer. They appear either at the orifice of the shell or 
upon a protruded segment of the entire body of the animal ; but 
they are also developed within the shell, at any part of the sur- 
face of the body. In the latter case they give rise to the ap- 
pearance of vacuoles and alveoli, which, however, are filled with 
sea-water and exist upon the surface of the body, and not within 
the central substance containing the vesicles. The elevations 
commence with an aggregation of contractile substance, small 
at first, at some part of the boundary of the contractile mem- 
brane ; they then gradually enlarge by the addition of more mat- 
ter from the surrounding parts, and the contractile membrane is 
seen to move over the central substance of the body containing 
the vesicles. By increase of the contraction, new elevations of 
various forms are sometimes developed upon a lamellar or 
elongated process, so that the originally membranous contractile 
lamella thus assumes variously branched forms. 
ce. The most slender kind of the elongated processes forms 
the so-called pseudopodia of the Polythalamia. These are most 
strikingly developed outside the shell, at the orifice ; but they 
exist also within the shell during the above-mentioned forma- 
tion of vacuoles. In the sarcodic net, as it is called, formed by 
them, membranous plates of the contractile substance are some- 
times so inserted, as shown by a described observation, that as it 
were a portion of the contractile substance, from which pseudo- 
podia are developed, maintains the connexion with the other 
parts of the contractile cortical layer merely by a slender pseu- 
dopodiform filament. The pseudopodia may arise directly from 
the cortical substance; but they are usually developed from 
stouter processes, in consequence of an increase of the contrac- 
tile action. The so-called granules observed in the granular 
movement must be regarded as very minute wart-like eleva- 
tions of the membranous contractile substance. They occur 
Ann. § Mag. N. Hist. Ser.3. Vol. xvu. 23 
