THE NEMERTINI I?9 



boscis of the Nemertines is uncertain. It appears probable that 

 a poisonous fluid is discharged into the wound made by the 

 armed proboscis a fluid secreted by the epithelium of the hinder 

 region of the proboscis. While in the Anopla the rods and 

 nematocysts possibly have some 

 stinging or numbing function. 1 



Eversion of the proboscis is 

 effected by the contraction of the 

 wall of the rhynchocoel, acting 

 upon the contained fluid. 



The rhynchocoel, which is de- 

 veloped as a cleft in the meso- 

 blast that forms around the in- 

 vaginating proboscis, is a closed, 2 

 cylindrical tube lying above the 

 intestine ; in Drepanophorus, alone, Fl - XVIL 



if- trivpc TM'CP fn a CPTM'PC nf Inner The annature of the proboscis of Drepano- 



U gives ] a Series < Ong, phorus crassus (after Biirger). 1, the curved 



narrow, non-mUSCUlar diverticula, ' edian acanthophore, with several stylets 



. . ill. i fixed upon it. 2, a transverse section of it ; 



Tight and left, Which Correspond , basis or acanthophore ; 6, stylet. 3, one 



-,i ,v A i- i of the numerous lateral acanthocysts. 



in position with the intestinal 



diverticula (Fig. XXI. w). The wall of the rhynchocoel (or pro- 

 boscis sheath) consists usually of an internal coat of longitudinal 

 muscle, and an outer coat of circular muscles, some of which in 

 Carinella are continued round the intestine. It is lined by a flat 

 epithelium, which is replaced by goblet cells over the dorsal vessel, 

 where this passes along its floor. The rhynchocoelic fluid contains 

 flat, ellipsoid, amoeboid corpuscles, frequently coloured by red or 

 yellow granules, possibly haemoglobin ; as a rule, the cells are 

 larger than the blood corpuscles, but vary in shape and size in 

 different genera. It is noteworthy that even in a "resting con- 

 dition," an attraction sphere is readily visible in them. 



This characteristic apparatus has no exact counterpart in the 

 Platyhelmia, although it resembles in its general anatomy and 

 mode of action each one of the four " proboscides " of Tetrarhynchus, 

 and still more, the complex rostellum of Drepanidotaenia. But 

 with these it can have no genetic relation ; the Nemertines have 

 probably been descended from some Turbellarian-like ancestor, and 

 among these the family Proboscidae appear to furnish a starting- 



1 It is worth noting that M'Intosh doubts both the fact that poison is secreted, 

 and the fact that the organ is one of aggression. There are curiously few actual 

 observations on the eversion of the proboscis. The matter requires investigation, as 

 also does the mode of action of the muscles of the organ. C. B. Wilson gives an 

 account of the employment of the proboscis in burrowing and locomotion ; see 

 " Habits, etc., of Cerebratulus lacteius " (in Q. J. M. Sci. xliii. 1900, p. 97), for many 

 interesting facts concerning the habits of this species. 



2 Biirger speaks, with considerable hesitation, of communications with the vascular 

 system in Cerebratulus. 



