THE NEMERTINI 191 



around the nucleus, etc. ; in these cases the sac seems empty when 

 the worm is mature, for the ripe egg cells push the wall outwards, 

 and come to lie in independent follicles in the parenchyma (Fig. 



XXL). 



It has been pointed out by Hatschek and by Mayer that each genital 

 sac presents all the usual relations and characters of a coelom, and these 

 sacs are the only organs which can be regarded as such. The excretory 

 system, at one time identified as coelom, is now known to be epiblastic 

 in origin (as in other worms) and to appear after the mesoblast has 

 formed, nor has the vascular system any claim, from developmental con- 

 siderations, to the title. The fact that the genital sacs appear late, and in 

 the simplest forms arise contemporaneously with the genital cells (as they 

 do also in Platyhelmia), cannot be regarded as a solid argument against 

 the view. It is, indeed, rather remarkable that in both these groups, 

 the lowest of the Coelomocoela, the coelom is an inconspicuous cavity. We 

 are so generally led to think that the coelom is a constant accompani- 

 ment of the mesoblast, that we forget that possibly the chamber is later, 

 phylogenetically, than its wall. Here, in Nemertines, when the gonads 

 are mature, there is a remarkable resemblance, in their repetition, in 

 their relations to other structures, to the coelomic segments of Annelids. 



The matter of " metamerism " is closely bound up with that of the 

 coelom. In the Annelids it is the mesoblast and its cavity that first 

 present repetition during embryogeny ; the internal segmentation 

 which exists in the Nemertines the repetition of the gonads, of the in- 

 testinal pouches, of the blood-vessels and nerves, and in Drepanopliorus of 

 the rhynchocoelic diverticula is essentially the same as in Annelids. 

 But in the Nemertines it is never accompanied by external marks of 

 metamerism, with the interesting and important exception of the genital 

 pores. There is no constriction of the body, no interruption in the 

 musculature of the body wall. 



It is a remarkable fact that the nephridia are not metamerically re- 

 peated ; it is true that in Ampliiporus and in Valencinia the nephridial 

 ducts and pores are numerous, but this repetition is quite irregular, asym- 

 metrical, and not coincident with that of other organs. This may perhaps 

 be explained by regarding the nephridia of Nemertines as homologous 

 with the "head kidneys" of Annelids, which appear early in ontogeny 

 and differ in structure from the metameric nephridia. We must believe 

 that these have not yet made their appearance, phylogenetically, in the 

 Nemertines. This fact is strongly opposed to Perrier's view that this 

 group is a degenerate descendant from Annelids, for we should then 

 expect to find traces of nephridia in the " trunk," where the gonads, etc., 

 are situated. The more usual view, that they are descended from the 

 Turbellarian stock, and have in some respects, and in some degree, 

 followed the Annelid line of evolution, is the more plausible view. The 

 condition of the nervous system fits in with this view specialisation of two 

 lateral tracts, which in the higher Annelids are destined to come together 

 and even to fuse on the ventral surface. 



The statement of Lebedinsky, too, that the mesoblast arises from two 



