REPORT ON THE NEMERTEA. 127 



ontogenetic development of Vertebrates. I will not circumstantially refute this argu- 

 ment, but will only remark that in Polygordius and other Chaetopods, which are 

 representatives of a group of animals in which segmentation reaches such a very hio-h 

 degree of perfection, the longitudinal muscular layer of the body-wall is as yet continuous 

 in the adult, and not divided into metameric sections, as it is in certain Arthropods and 

 in Vertebrates. Now let us consider contractions of the inner muscular layer a of the 

 Nemertea, the only layer that is common to all of them, from Carinella to Cerebratulus 

 and from Cephalotkrix to Pelagonemertes. This layer also corresponds with the longi- 

 tudinal muscular layer just alluded to of other lower worms, such as Polygordius, and, 

 as was noticed in our paragraph on the muscular system {cf. p. 72), its contraction is 

 sometimes very distinct in favourable sections. 



We then see the contraction marked out as so many successive blocks of contracted, 

 thickened fibres, separated by intervening parts of non-contracted fibrous tissue (PL XV. 

 figs. 9, 10). The sections demonstrate that the phenomenon persists throughout the 

 whole breadth of the animal, i.e., that successive rings of contractile tissue alternate with 

 intervening rings in which no contraction is observed. This phenomenon is thus in a 

 certain degree comparable to an arrangement in distinct myomeres. 



It is not unimportant that it was especially noticed in the fundamental muscular 

 layer, and it may at the same time be remarked that it appears, from what I have as yet 

 been able to observe myself, that the number of these rings in a given length of the 

 animal, is the same, or a multiple of the number of intestinal caeca and transverse 

 nerve-tracts in the plexus ; in other words, that the incipient metamery of the internal 

 organs is in a definite relation to these phenomena — which might also deserve the name 

 of incipient metamery — in the muscular layers. 



For the present the fact is, however, not yet definitely demonstrated that these 

 successive blocks are indeed present as such in the living animal. The possibility is still 

 open that they may be waves of contraction which have been fixed at the moment of the 

 immersion of the animal in the preserving fluid. For this reason I will not lay any 

 undue weight on this observation. 



The ideas concerning the origin of metamery here expressed, and advocated for 

 several years in my university lectures, differ from those of Lang (XVIII) and 

 Sedgwick, 1 in so far as they do not recognise the primary importance of the so-called 

 ccelomic sacs — the paired archenteric diverticula of Amphioxus — for the solution of this 

 question. 



The question of the Vertebrate ccelome, so full of obscurities and difficulties, is 

 purposely left out of consideration here, where the relation to archiccelous ancestral 

 forms is discussed, and where an attempt is made to show that it is indeed probable that 

 the impulse towards the establishment of metamery is due to forces for which the 



1 A. Sedgwick, On the Origin of Metameric Segmentation, Quart. Joum. Micr. Hci., vol. xxiv. p. 43, 1884. 



