BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 347 



the proboscidian sheath, and that, too, in a definite manner, since no great devia- 

 tion hi a series of specimens is met with. 



In the developing organ of 0. gracilis, a very good analysis of the somewhat 

 complicated structure is obtained, so that doubtful anatomical points are cleared 

 up satisfactorily. The sac at the base of the central stylet is sometimes seen to 

 be composed of granules in rounded masses ; and they are all grouped posteriorly 

 at an early stage, and thus present a similar form to that seen in other species 

 which have no such elongated sac in the complete state. It is curious to witness 

 the accuracy with which the stylets are reproduced in this and other species. 

 There is never any confusion, but each invariably produces them of their respec- 

 tive sizes and curves as infallibly as if they had been struck out of the same 

 mould. Yet these bodies are not in any way organically connected with the 

 tissues of the proboscis, but only spring from a secretion poured into the lateral 

 sacs, or from the central apparatus. In the concentric arrangement of their con- 

 stituent substance, and some other particulars, these spicula are analogous to 

 those of the sponges, whose microscopic anatomy has been so excellently investi- 

 gated by Dr Bowerbank* Indeed, the morphology of the stylets of the Omma- 

 topleans offers elements for deeper reflection than even the hooks and bristles of 

 the higher annelids, which are often so diagnostic of genus and species. 



Besides the developing organ, the proboscidian chamber contains (unless in 

 cases where the organ has been violently expelled) the cast-off proboscis ; and it 

 is a curious sight to observe a fully-developed organ floating freely in the chamber, 

 and still endowed with contractile power, while the new proboscis has advanced 

 to the stage of the advent of stylets. The discarded organ soon becomes opaque, 

 appearing reddish by transmitted light, and the stylets leave their positions. As 

 there is no mode of exit after the new proboscis has begun to develop, the aborted 

 one can only (not to speak of rupture) be removed by disintegration and absorp- 

 tion ; and hence in the proboscidian chambers of such animals there is a vast 

 increase of cells, granules, and granular debris. 



Digestive System. — Though no such transverse muscular plate, as described by 

 M. de Quatrefages, occurs at the anterior part of the body of the worm, yet there 

 exists a very distinct and comparatively large ciliated oesophageal chamber or sac, 

 as first described by Sig. Delle Chiaje, apparently in a Borlasian.f The figures of 

 the supposed transverse plate given by the former, indeed, show some degree of 

 doubt, since in the large figure J both wavy and longitudinal fibres are represented, 

 while in the small figure there are only transverse fibres. I fear the wavy longi- 

 tudinal lines owe their presence to those actually existing in the oesophageal sac. 

 Dr Johnston's figure § of 0. 7nelcmocephala indicates this structure, to which he thus 



* Monograph of the British Spongiactee, Ray Society, vol. i. p. 5, et seq. 

 f Memorie sulla, &c. vol. ii. 1835. % Op. cit. pi. xix. fig. 1, m. 



Op. cit. pi. ii. a fig. 5.* 



