8 DISCOVERY REPORTS 



In addition to these muscles which have their homologues in Nebolia, Nebaliella has a pair of very 

 strong muscles (t.p.m.) which stretch ventro-laterally from a thickening of the chitin of each lateral 

 lobe of the trilobed process immediately anterior to the point where the process splits from the 

 gut wall. 



THE STRUCTURE OF THE GUT OF NEBALIOPSIS TYPICA (Sars) 



FORE-GUT 

 The external features of Neboliopsis (Cannon, 193 1, pi. xxxii) indicate that it is a highly specialized 

 member of the Nebaliacea, and this is confirmed by the internal organization. Even in the gastric 

 mill group resemblances are few. 



Fig. 4 shows the reconstruction of the right half of the fore-gut in three sections. 



The molar processes of the mandibles are reduced and do not project into the mouth as in the 

 other forms. The mouth is a transverse slit between the labrum (/.) and the paragnaths {pa.). These 

 can be retracted by strong muscles, thus uncovering a flat plate of chitin with a median antero- 

 posteriorly directed slit leading into the stomach. The latter slit can be opened widely by the dilator 

 muscles [l.dil.), which slope upwards and outwards from the oesophageal wall. Very great variation 

 of both the size and the shape of the gape is thus possible by the combined action of these two slits. 



There is no distinct separation into oesophagus, cardiac and pyloric parts, but the region surrounded 

 by the horizontal circular muscles, and to which the lateral dilators are attached, may be considered 

 oesophageal in comparison with Nebolia. 



Setae are entirely absent from the gut. Except in the most anterior and dorsal regions the chitinous 

 lining of the lateral walls has the appearance of crazy-paving owing to the presence of grooves over 

 the junctions between the individual cells of the supporting tissue (Fig. 5 C, D). Posteriorly these 

 grooves are less distinct. The paved parts of the walls lie very close together (Fig. 5 C) and provide 

 a good gripping surface. 



An anterior median projection (a.m.p.) is present as in Nebalia, but much reduced. There is a dorsal 

 ridge [d.r.) which is strongly chitinized and slightly grooved in the region immediately dorsal to the 

 mouth, but it arises anteriorly as a soft pad and becomes so again posteriorly. The lateral walls have 

 thickenings {l.th.) against which this ridge bites. The thickenings of the chitin are prolonged into short 

 anterior projections of the stomach, and may be homologous with the slight thickenings at the bases 

 of the spines (^-^.2) in Nebalia. There are no grinding tubes, but a strong grinding or biting action 

 probably occurs between these heavily chitinized regions. 



Posteriorly where the dorsal ridge becomes a soft pad, the side walls open out slightly and they also 

 become soft. The chitinous lining of the fore-gut ends raggedly ; the walls become glandular and lose 

 the thick muscle sheath which encircles them throughout the stomach region. 



MID- AND HIND-GUT 

 The glandular region mentioned above marks the beginning of the mid-gut. Here the anterior 

 digestive diverticula (d.gl.), which are comparatively small (Fig. 6 C), and extend only a short distance 

 forwards and which are probably homologous with the dorsal caeca of Nebalia, open by irregular 

 apertures. Some of these are small channels passing through the glandular region, but the largest 

 opens below a flap (gl.r. in Fig. 4) directly into an immense digestive sac {d.s. in Fig. 7 A), which 

 widens out suddenly and almost completely fills the body cavity back to the end of the fourth abdominal 

 segment, and which may be homologous with the digestive caeca of Nebalia, though it is difficult to 

 be certain of any homologies when the specialization is so great. It is important to note that this is 

 a plain sac without any convolutions and with only a few thin septa rising from its walls. The enlarge- 

 ment therefore does not provide a great deal of extra surface area for absorption, but it does give a 



