AFFINITIES OF ECIIIURUS UNICIXCTUS. 85 



nerves do not originate accui'ately opposite one anotliev ; they spring from the ventral 

 region of the main cord, and are composed only of nerve-fibres. Spengel found in 

 Echinrus Pal/asii that tliese lateral nerves form definite rings or commissures in the body- 

 wall, as nerves from opposite sides meet in the middle dorsal line — their course being, 

 according to him, directly under the roAvs of papillae. In Echinrus uaicinctus the nerve- 

 cord is held in place by its side branches, there being no connective-tissue attachment, such 

 as is found in other forms, Rietsch, in describing the nervous system of JBonelUa minor, 

 remarks: " Le systeme nerveux se compose du cordon ventral depourvu de renflements 

 ganglionaires, et fixe aux teguments par un mesentere." Though it does not agree with 

 this form of attachment, yet it is not seated directly on the body-wall as in Thalassema 

 nepkmi, of which Eietsch observes : " Ce tronc nerveux n'est pas, comme chez la 

 Bonellie, I'elie aux teguments par un mesentere ; il repose directement sur la muscula- 

 ture." The cord is, in Echiuriis unicinctus, free from th(» body-wall, except in so far as 

 its fine lateral branches fix it in position, by becoming embedded in the musculature. 



0\vin<r to the contraction of the bodv-wall. the central nerve-cord in these preserved 

 specimens is thrown into wavy folds (PL 7. figs. 5, 12 ; PI. 9. fig. 27, u.) ; this condition 

 makes it easy to see that the cord is free of the body-wall but for the attachment of 

 the nerve-branches. 



Anteriorly the cord bifurcates (PI. 7. fig. 12 ; PL 9. figs. 23, 24, 27) ; the two limbs, 

 passing round the buccal cavity, eater the substance of the proboscis, at the tip 

 of which they meet, thus forming a true circum-oesophageal ring. In I'orms such as 

 E. Fallasii, or BoneUia, where the proboscis is a very extensive organ, this nerve-loop 

 is necessarily much elongated. But in the species under consideration, the proboscis 

 being scarcely more than a prostomial lobe, the nerve-ring is almost as compact as in the 

 Earthworm (PL 9. fig. 27). 



Similarly, at the posterior end the cord splits into equal halves ; and these enter into 

 close relation with the wall of the rectum. It is not easy to follow these two branches, 

 but they apparently divide up into finer ramifications which supply the muscles of the 

 anal circlet of sctoe. 



There are no ganglia in the A"entral cord. 



The outline of the cord in transverse section (PL 9. fig. 28) is seen to be rather flattened 

 dorso-ventrally, and is not circular as in Echiurns Pallasii {vide Greeft', Spengel). The 

 cord has a coating of nerve-cells {c), most marked dorso-laterally — ventrally the cellular 

 elements are fcAver and more scattered. These nerve-cells vary much in size; they 

 enclose a central mass, made up of ramifying nerve-fibres (/.//.), the majority of which 

 run longitudinally as is seen in longitudinal sections (PL 9. lig. 30, f.n.). Many of 

 the fibres constituting this central fibrous mass are not nervous, but are inward 

 extensions of the protective connective-tissue sheath which covers the cord. The nuclei 

 which lie scattered throughout this fibrous area may belong to these connective-tissue 

 elements, or to the nerve-fibres, but from the preserved material it is impossible to iorm 

 a conclusive opinion on this point. In this outer sheath, the longitudinal muscles are 

 much less conspicuous than in similar sections of the nerve-cord of Echiurm Pallasii, 

 or of Lumhricus (PL 9. figs, 31, 32, sh.). 



SECOND SERIES.— ZOOLOGY, VOL. VIII. 13 



