ENTKROPNEUSTA FROM THE ABROLHOS ISLANDS. 89 



the genital pleura. The most anterior large saccules are much darker in 

 colour than those following, the posterior saccules being but little different 

 in colour from the pale yellow of the body generally. 



Caudal Region. — This region varies greatly in length. It is delicate and 

 often breaks off, so that it is frequently missing in preserved specimens. 



A very delicate ridge can just be traced running along the mid-dorsal 

 line, corresponding in position to the dorsal nerve-trunk. Parallel to this, 

 and slightly more distinct, are two ridges (one on either side of the mid- 

 dorsal line) which are continuous anteriorly with the genital pleura. In the 

 preserved specimens the entire animal — from the postbranchio-genital region, 

 at least, to the end of the caudal portion is marked by close annulation. 



Anatomy. 

 Proboscis. 



The epidermis of the proboscis (PI. 10. fig. 2, Epid.') varies between '10 and 

 •18 mm. in thickness. A conspicuous nerve-fibre layer lies at its base and is 

 almost one-fifth to one-quarter the total thickness of the epidermis. There 

 is a thin basement-membrane below the epidermal layer, and underlying 

 this a conspicuous, although delicate, layer of circular muscle-fibres^ which 

 reaches its greatest development towards the base of the proboscis (PI. 10. 

 fig. 2, & PI. 11. fig. 5). Between the nerve and circular muscle-layers is a 

 prominent system of blood-lacunse — the vascular network of the proboscis. 



The Longitudinal Muscle Fibres of the proboscis are arranged, as is usuallv 

 the case, in radial bundles (fig. 5, JRad.M.), which extend distally to the end 

 of the glomerulus and are marked out by radial fibres. These bundles touch 

 as they near the centre of the proboscis. At about the level of the glomerulus 

 the longitudinal muscle-fibres encroach upon this central organ. More 

 proximally, however, the muscles leave a space — the proboscis coelom 

 (figs. 2 & 5, Pr.Co.) — which surrounds the stomochord, glomerulus, etc.^ 

 the organs forming the central complex of the proboscis (figs. 2, 4, & 5). 

 This proboscis coelom becomes divided dorsally, and towards the neck of 

 the proboscis, into two pouches, which lie to right and left of the middle line 

 (fig. 5, Dor.P.C). As in other Ptychoderidse the division is due to the 

 presence of the pericardium (fig. 5, Per.) . The two halves of the proboscis 

 coelom so formed dorsal to the central complex open generally to the exterior 

 at the base of the proboscis by two dorsal and equal proboscis-pores (PL 11. 

 figs. 8& 9, P.jo.). 



The ventral portion of the proboscis coelom is continued posteriorly into a 

 kind of csecum (fig. 2, V.p.c), which also becomes separated into right and 

 left halves — in this case by a septum, which becomes more distinct and better 

 developed as one passes posteriorly (figs. 6 & 8, V.p.s.). Both the anterior 

 and posterior edges of this septum are free, so that transverse sections 

 through the extreme end of the ventral proboscis csecum show no septum. 



