BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 379 



this chamber and other points agree so closely, both structurally and function- 

 ally, with the same parts in Ommatoplea, that it is unnecessary to describe 

 farther than refer to the aspect of the parts in the living animal (Plate X. fig. 

 l,o); and to the various transverse sections, in which the wall of the chamber is 

 lettered 0, and the cavity ao. Sometimes near its diminished posterior end the 

 latter shows a series of moniliform spaces, from internal bridles, and often does 

 not quite reach the tip of the tail either in this group or in Cephalothrix. In 

 Meckelia annulata the proboscidian sheath is not continued to the tip of the tail 

 either, and it is an interesting fact that this absence coincides, as in the last-men- 

 tioned genus, with greatly enlarged lateral vessels. In Cephalothrix the chamber 

 presents certain peculiarities, being subdivided by transverse bands of contractile 

 tissue throughout its entire length, so that during the motions of the worm the 

 anterior region is occasionally thrown into a series of moniliform spaces. These 

 contractile septa (though imperfect in the middle), doubtless prove of much ser- 

 vice during rupture — an occurrence so liable in this lengthened animal. More- 

 over, the wall of the chamber is thin, and the circular muscular fibres of the 

 body not much developed ; hence the advantages afforded by these safeguards 

 against the inconvenient bulging of the chamber during the motions of the worm. 

 The transparent liquid of the cavity in this genus (Cephalothrix) contains flask- 

 shaped bodies and minute clear corpuscles. 



Prof. Keferstein* seems to have had no definite idea of this chamber as a 

 cavity with special muscular walls, but speaks of the peculiar discs as floating 

 in the body-cavity (Leibeshohle)— an error of some importance. In his two trans- 

 verse sections of Cerebratulus marginatus, he appears to have confounded the 

 wall of the tunnel with that of the proboscis. He is thus less correct than his 

 predecessors Frey and Leuckart,| who noticed the sheath of the proboscis and 

 its contents. 



Terminal Aperture in the Snout for the Proboscis. — A channel, ciliated for some 

 distance, leads inwards from the terminal pore to the reflection of the proboscis 

 just in front of the commissures. This channel, shortly after its commencement 

 (Plate X. fig. 4, a) , is surrounded by an elaborate series of muscular loops 

 (indicated at 2), which, while keeping it closed under ordinary circumstances, 

 permit of rapid and easy dilatation. Immediately within these is a series of 

 longitudinal muscular fibres, which attain a more distinct development some- 

 what posterior to this point («, Plate XII. fig. 2). A very beautiful group of 

 circular and diverging fibres lies to the outside of the first-mentioned series (2, in 

 the last-mentioned figure), crossing each other in a striking manner superiorly 

 and inferiorly, as well as less distinctly at intermediate points, and forming with 

 the longitudinal and other fibres the intricate stoma of the snout. The terminal 



* Op cit. pp. 68 and 69. f Beitrage zur Kenntniss, &c. p. 70. 



