WITH NOTES ON THE WEST INDIAN SPECIES. 233 



Behind the coecal region the stomochord quickly loses its chorda-like character, 

 and no doubt also its rigidity ; its function as a supporting organ being, in this region, 

 entirely usurped by the nuchal skeleton. It tends, at the commencement of the nuchal 

 region, to diminish in size up to a certain point, always, however, maintaining its in- 

 tegrity and its lumen (PI. XXVIII. Fig. 3). Its ventral wall becomes progressively 

 thinner, and, some distance in front of the bifurcation of the nuchal skeleton, the 

 stomochord begins to widen out into the characteristic flattened terminal portion, which 

 finally opens freely into the buccal cavity. 



Nuchal Skeleton. 



This structure is sometimes referred to as the collar-skeleton, sometimes as the 

 proboscis-skeleton, and Spengel calls it the Eichelskelet. 



As the mass of it lies in the narrow neck which joins the proboscis to the collar, 

 while the cornua lie in the collar and keep the mouth permanently open, the above 

 seems to be the most appropriate designation. 



There are at least six features of the skeleton to be taken into account, namely, 

 (1) Cupule (Endscheibe or Trichter of Spengel), (2) Body, (3) Crest, (4) Alary pro- 

 cesses, (5) Carina or Keel (Kiel or Zahn of Spengel), and (6) Comua (Schenkel of 

 Spengel). 



The cupule embraces the posterior end of the ventral coecal dilatation of the 

 stomochord (PI. XXVIII. Fig. 2). In Pt. flava it is quite shallow, passing abruptly 

 into the solid body of the skeleton. The body in its anterior portion is massive, 

 laterally compressed, sub-rectangular, and produced along the dorsal middle line into 

 a prominent acuminate crest, which projects into the ventral wall of the stomochord. 

 The crest varies, however, in the degree of its development and is sometimes obsolete. 



In the mid-region of the body of the skeleton, an accessory skeletal element is 

 added to the main body in the form of a transverse arcuate bar produced at the sides 

 into alary processes, and embracing the dorsal side of the ventral coecum of the 

 proboscis. 



In fact, in Pt. flava, as in Pt. erythraea described by Spengel, there is no keel 

 along the ventral side of the nuchal skeleton, its place being occupied by the 

 enlarged ventral coecum of the proboscis 1 . Behind the alary processes the body of the 

 skeleton becomes subtriangular in section, the base of the triangle being directed 

 dorsally and the apex ventrally (PI. XXVIII. Fig. 3). Immediately in front of the orifice 

 of communication between the stomochord and the buccal cavity, the body of the 

 skeleton becomes bisected, and from this point the cornua begin to diverge. 



As in all species of Ptychodera the cornua, which lie on each side in a fold of 

 the epithelium of the throat, do not extend backwards beyond the middle of the 

 collar-region. 



of the dilated region of the stomochord appear as distinct lateral pouches and are described below under that 

 designation (cf. PI. XXX. Fig. 25). 



1 For full treatment of the nuchal skeleton in its capacity of derivative of the basement-membrane, with 

 proof of its dual origin from stomochord and epidermis as shown by the lines of stratification, etc., see 

 Spengel's Monograph, p. 487 et sec;. 



33—2 



