98 PROF. W. J. DAKIN ON A NEW SPECIES OF 



states that it commences as " a simple thickening of the ventral wall of the 

 hind gut, which is soon drawn out into a flattened band with dilated distal 

 (ventral) border." 



In Pt. pelsarti a pygochord is present, and consists, for the greater part, of 

 -a band of tissue running from below the gut-epithelium in the mid-ventral 

 line to the basement-membrane of the integument. The tissue is cellular 

 with oval nuclei, and on each side of it is basement-membrane. The 

 ■structure certainly seems too delicate to be of much service as a support, 

 although it may possibly serve as a kind of mesentery. There is no evidence 

 in the species examined either in favour of or against Punnett's ingenious 

 suggestion that the pygochord may be the remains of a ventral siphon now 

 Testigial. 



Summary. 



It will be advisable perhaps to add, in the form of a summary, a short 

 diao-nosis of the characters of the species Ptycliodera pelsarti. 



Ptychodera pelsarti, n. sp. — Moderately large form, average size of 

 preserved specimens 50-60 mm. (considerable contraction had taken place). 

 Proboscis cavity with longitudinal muscles gathered into distinct radially 

 arranged bundles. JSeck of proboscis with a distinct well-developed but 

 unlobulated " racemose" organ. Ventral proboscis, csecum, and " racemose'" 

 organ compressed laterally. Cornua of nuchal skeleton very long and 

 reaching posterior half of collar^ body of skeleton with characteristic shape. 

 Two proboscis pores. Stomochord with well-developed lateral pouches. 

 (Esophageal region of pharynx predominates over the branchial. Collar 

 nerve-cord with continuous lumen and usually with three dorsal roots. 

 Locality : Pelsart Island, Abrolhos Islands, West Coast of Australia. 



EXPLANATION OF PLATES 10 & H. 



Reference letters. 



(Figures 1-4, 6 are ou Plfite 10 ; fig'ures 5, 7-12 on Plate 11.) 



Ant.Np Anterior neuropore. Fh^. 2. 



Br Branchial bar. Fig. 11. 



Bj-,c Branchial region of alimentary canal. Fig. 11. 



C.b.s Central blood-sinus of proboscis. Figs. 2, 4, 5, 8, & 9. 



c.m.c Circ. muscle-fibres of collar. Fig. 2. 



C.tis Connective tissue of collar. Figs. 2 & 10. 



Can.N.C. .... Canal of dorsal nerve-cord. Figs. 2 & 10. 



Ch.T Chondroid tissue. Figs. 7, 8, & 9, 



Coll.Co Collar coelom. Figs. 2 & 10. 



D. C.c Dorsal ccelomic canal of proboscis. Figs. 2, 5, & 9. 



D.N.C. Dorsal nerve-cord. Figs. 2 & 10. 



