376 DR W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



spiral manner like a cork-screw or the stalk of a Vorticella, or twist their bodies 

 into a rope of various strands. The great Lineus longissimus may now and then 

 be observed in its native pools extended between the Fuci of opposite sides in 

 numerous loops, each several yards in length, and so intricately arranged, that 

 they can scarcely be unravelled by other than the animal itself. The extreme 

 stretching which the body undergoes before it snaps — as in attempting to secure 

 a specimen in an intricate and inaccessible pool — and the extraordinary shorten- 

 ing on immersion in spirit, are only well-marked conditions into which the animal 

 throws its yielding textures at will. A Micrura, again, from the deep water of 

 St Andrews' Bay, swims freely on its edge like a fresh- water Nephelis, or its own 

 ally O. pulchra, lashing the water with alternate strokes of its muscular and flat- 

 tened posterior extremity. Sir J. G. Dalyell likewise noticed this edge-motion 

 in his great " Gordius' n fragilis, but he was not sure whether it was a natural 

 condition, or caused by the confined vessel. Meckelia annulata forms in captivity 

 a beautiful silky sheath by its cutaneous secretions, within which it lies in com- 

 parative security, until, tempted perhaps by love of change, it searches for a fresh 

 site, whereon to manufacture a new chamber for its protection. In unhealthy 

 and slowly dying animals the skin becomes raised into pale bulla?, not only from 

 corrugation, but from degeneration of the cutaneous textures. 



The posterior end of the body in Micrura(Stfjlus) requires special mention, since 

 there is superadded a peculiar elongated and contractile style. This appendage 

 seems to be formed by a prolongation of the cutaneous and part of the muscular 

 (longitudinal and circular) textures of the body-wall of the animal. The entire 

 organ in contraction has a granular appearance, the coarsest granules, and occa- 

 sionally a few circular masses of brownish pigment, being at the tip. Within 

 these coats is a central chamber, which undergoes various alterations in size, and 

 contains a transparent fluid. This cavity is not connected with the digestive 

 tract, which opens by a terminal pore at the base of the process, nor can pro- 

 boscidian discs be seen therein. I have not as yet ascertained with w T hat system 

 it communicates, but its connection with the circulatory appears most probable. 

 The style is richly ciliated externally, and undergoes many and varied motions, 

 now forming a verrucose knob, now stretched to an extreme degree of tenuity, 

 and apparently assisted in the latter action by the fixing of the tip, whose warty 

 formations seem to perform the functions of suckers, for the animal may be 

 observed crawling about with a loose style, then the tip of the latter suddenly 

 becomes fixed upon the clean and smooth glass, and the whole organ is elongated 

 accordingly. The fixed portion at the tip is usually more dilated than the suc- 

 ceeding part of the style. 



In Cephalothrix, (Erst, (including Astemma), the dermal tissues, and indeed 

 the entire body-wall, deviate from the ordinary structure in Ommatoplea, Bor- 

 lasia, and Meckelia ; and while the minute anatomy of this genus bears out the 



