BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 333 



walls of the proboscidian sheath, rather behind the middle of the animal, the 

 fibres spreading out in a fan-shaped manner, and mingling with those of the tube. 

 The motions of these muscular bands is most interesting, now jerking into nume- 

 rous graceful folds or coils, by a sudden contraction, like the stalk of a Vorticelta 

 now shortening more gradually — the curves being thickened here and there by 

 the bulging of the fibrillse. They are simply muscular fasciculi, with very fine 

 longitudinal lines — the marks of the fibrillar, and seem to restrain the irregular 

 protrusion of the proboscis and assist in its retraction. This muscular arrange- 

 ment is also the ultimum moriens, showing contractions when all other signs of 

 life have fled. In a young Tetrastemma variegatum, in which the riband had 

 been ruptured from its attachment, the fibres (Plate VI. fig. 6, ^) had assumed 

 a clavate aspect from contraction, and only very faint longitudinal markings were 

 visible. 



Before reviewing the statements of previous investigators with regard to the 

 general structure of the foregoing parts, a description of the peculiarities of the 

 regions in other species of Ommatoplea will be narrated. 



In Ommatoplea melanocephala (Johnst.), the proboscis is somewhat larger in 

 proportion than in 0. alba ; and, while the type of structure is adhered to, there 

 are several important differences in detail. The stylet-region (Plate VI. fig. 7) is 

 peculiar in having the lateral stylet-sacs carried considerably forwards, so that 

 they lie quite in front of the central apparatus, and the floor of the anterior 

 region has consequently to form a deep pit to reach the spike of the stylet. In 

 this figure the organ is shown comparatively free from pressure, and the encroach- 

 ments of the lateral sacs on the cavity may thus be correctly estimated. The 

 basal sac of the central stylet is proportionally large, while its wedge-shaped 

 setting is comparatively meagre. The powerful series of oblique or radiating 

 fibres which pass outwards and forwards from the latter, in the usual position of 

 the organ under pressure, are very distinctly shown, and, as it appears, sling the 

 apparatus. The points of the stylets (central and lateral) are rather blunt, and 

 their shape, on the whole, resembles that found in Tetrastemma algas. Some of 

 the looped muscular fibres of the reservoir seemed to pass inwards beyond the 

 exterior ring in front, so that a continuous series of fibres would thus be formed, 

 as in certain viscera* (bladder, &c.) of the higher animals, and the chamber 

 environed with the exception of the anterior and posterior openings. The circlet 

 of granular glands is much developed in this species, and often renders the subjacent 

 parts obscure. 



The remarks and figures of M. de Quatrefages f relating to this species (his 

 Polia coronata) require amendment. He mentions that it is the only exception 



* Vide the admirable Researches of Dr Pettigrew, Philos. Trans, part ii. 1867. 

 \ Recherches Anat. &c. p. 166. 



VOL. XXV. PART II. 4 Q 



