52 . ANATOMY OF THE ENOPLA. 



as far as the stylet-region. The entire organ is proportionally larger than in Linens, and its 

 anatomy more apparent ; though I very much doubt, even in this group, if we can assign 

 it the ideal office of a vertebral column. At the point of reflection there is sometimes a 

 kind of os (Plate XV, fig. 4, a), from the slight folding of the lips of the organ in the early 

 stage of ejection. The muscular fibres are chiefly longitudinal at the commencement, and if in 

 a partially protruded proboscis a section be made in front of the ganglia (Plate X, fig. 9), the 

 difference between this and the succeeding reticulated portion is very conspicuous. Occasionally 

 the organ assumes a twisted position under examination, so as to give the fibres a spiral appear- 

 ance (Plate XVI, fig. 1, a), and in such a state the structure might fancifully be likened to the 

 spiral arrangement of the muscular fibres in the oesophagus of certain of the higher animals ; but 

 this condition of the proboscis is purely accidental. I fear, however, it has led M. de Quatrefages 

 into an erroneous interpretation of the anatomy of the organ in Folia glauca, the proboscis of 

 which is described and figured as having regular spiral belts at its commencement. The same 

 author again avers that in Folia mutabilis the latter portion consists of two longitudinal muscular 

 coats separated from each other by a cellular layer, a provision, he explains, for enabling each 

 to act independently. He also adds that no circular fibres were seen in this species, in 

 P. filum and some others. Dr. Johnston, on the other hand, considered the organ to be 

 homogeneous. 



In very small specimens of the British examples the transparency of the tissues of the proboscis 

 renders definition of the coats somewhat obscure, especially after mounting in chloride of calcium; 

 but, so far as I have observed, the structure is as follows : — Externally is a layer of elastic 

 tissue (Plate XI, figs. 4, and 9, g), which is more distinctly striated in transverse than in longi- 

 tudinal sections. Towards its free border, also, certain obscure granular markings observed in 

 the latter sections (Plate XI, figs. 7 and 8, g) show that the course of the external fibres is 

 different from the others ; indeed, in some views, the appearance is such as to raise the suspicion 

 of the presence of the ends of a few fine muscular fibres, the rest being nearly homogeneous. 

 The next layer is a somewhat narrow belt of longitudinal muscular fibres (/), which may be 

 termed the external longitudinal muscular coat. It consists of pale, unstriped, muscular fibres. 

 Between this and the other longitudinal layer is a remarkable stratum, the reticulated coat (e), 

 which in transverse sections (Plate XI, fig. 4) assumes a regularly moniliform appearance, from an 

 increase of its constituent substance at certain points. In many longitudinal sections (e.g. Plate XI, 

 fig. 8) the ends of numerous fibres are found in this layer, as if it was composed of circular 

 fibres ; but the appearance is due to intermediate bands which pass between the thicker longitudinal 

 columns. If a thin longitudinal slice from the organ in Amphiporus pulcker is hardened and 

 mounted in chloride of calcium, numerous well-marked homogeneous longitudinal belts are seen 

 at regular intervals, and between them are many connecting transverse fibres. The ends of 

 the fibres in these sections have therefore been caused by the knife severing the latter 

 series. Thus the tube is surrounded by a complete investment of this elastic meshwork, 

 which, doubtless, has an important physiological bearing on the varied movements of the 

 organ. In the Amphiporus grcenlandicus of CErsted, the longitudinal belts of this layer 

 are somewhat rounded in transverse section. The succeeding stratum (d) consists of a 

 powerful series of longitudinal fibres, fully twice as thick as the external longitudinal layer, 

 and which may be termed the inner longitudinal muscular coat. In essential structure it 

 resembles the exterior, differing only in bulk. In sections prepared by hardening in alcohol, 



