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tilirils arc. liowcwr iiitimatcly, iiitciian-d with each other aiul lietvveen tlic lonyituiHiial and transversal nervous-tul)es, and have, 

 thus, a very cinnplex course which, in transverse sections, gives rise to a spongious appearance. The longitudinal tubes 

 are, principal!}-, situ.ited in the inner and dorsal part of the longitudinal commissures, where, also, the\- have a large diameter, 

 vide PI. \', hg. 2, 12 and lO. Some tubes, also, pass along the outer side of the commissures where the nerves are given 

 oti'. \ide PI. \', tig. ; and 6. Some of the longitudinal nervous-tubes pass into the æsophageal commissures (PI. V, fig. 2 and 

 3) and bring, thus. I think, the æsophageal ring and the cerebral ganglia into communication with the various parts of the 

 \entral cord. The result of my observations, upon the histological structure of the ner\ous system of the Myzostomida, may 

 lie summed up in the following laws, which probabl}-, also, to a great extent, apply to the structure of the Annelids and 

 Arthropods. — i- Each of the ganglionic cells, whether unipolar or multiptjlar, has, as a rule, only one true nervous pro- 

 longation. ( )f these cells there are two t)-pes; one t^pe, the prolongation of which passes, directly, into one of the peri- 

 pheric nerves and forms ner\-ous tubes; another type, the prolongation of which loosens itself up into the fibrillar reticulation. 

 From the prolongations passing directly into nerves, fibrils are given oft' to the fibrillar reticulation. — 2) Most of the gang- 

 lionic cells of one side of the ventral cord send their prolongations into the peripheric nerves of the opposite side; some of 

 them send their prolongations to the lilirillar reticulation of the longitudinal commissure of the opposite side. — 3) Some "^an^lio- 

 nic cells send their prolongation, either, into peripheric nerves on the same side as they themselves are situated, or into the fibrillar 

 reticulation ot the commissure on the same side as they themselves are situated. — 4) Each peripheric nerve receives its nervous 

 tubes, either, directh" from the ganglionic cells, or, from the fibrillar reticulation of the longitudinal commissures. — 5) The long- 

 itudinal commissures are composed of ai longitudinal (and transversal) nervous tubes, of which, some of the longitudinal ones 

 pass into the oesophageal commissures; the purpose of the longitudinal tubes is, I think, to place the various parts of the 

 central nervous system in communication with each other; b) the fibrillar reticulation, consisting of slender, intimately inter- 

 laced tubes or fibrils, issuing from the nervous tubes (the prolongations of the cells, the longitudinal tubes, the tubes of the 

 intermediate nerve) or, from cells vviiose prolongations loosen up into such fibrils. — ()) Each nerve receives nervous tubes 

 having communication with the longitudinal tubes, as well as with the fibrillar reticulation. — 7} Branchlets or fibrils are 

 given olit" from the nervous tubes to the mass of the commissures; this can be, most distinctly, seen in the tubes of the in- 

 termediate nerves, from which branchlets are given ofi", alternately, to the commissures. I have, also, distinctly, observed sim- 

 ilar branchlets issuing from the prolongations ot the cells. 



The development of the nervous s}-stem. We know but \-er}- little about the development of the nervous 

 system. Beard describes a ;Tunctional larval < nervous system, which he believes completely disappears »in the later stages- 

 of larval life < and has nothing to do with the formation of the adult nervous system '<, as he has failed to find any traces of 

 an esophageal ring in the adult. .U'dging from the precedmg description of the esophageal ring. Beard's description 

 cannot be accepted as correct; the median thickening of the epiblast of the apex of the praeoral lobe is, I believe, 

 the true oesophageal ring and cerebral ganglia, in their initial development. Subsequently comes, the development of the ven- 

 tral cord. We have, thus, in the Myzostomida, a development of the nervous system, quite similar to the t\'pc common to 

 the Annelids. 



Abnormit}- in the structure and dex-elopment of the nervous system. I ha\'e frequenth' observeil this. 

 I have, especially in M. graffi — partly, also in M. gigas — met with specimens in which the ventral nerve-cord has a re- 

 markably small development, whilst the rest of the nervous system did not appear to be \ery abnormal. PI. II, fig. 7 

 illustrates the transverse section ot such a specimen ; b.s. is the trans-sected ventral ner\-e-cortl, posessing a flattened broad 

 form. The ilifterence between this abnormity and a normal ventral cord, such as is illustrated in PI. 11, fig. 15, is remark- 

 able. There is but little structure visible in such degenerated cords. Assymmetry in the structure of the nervous s\-stem. 

 especialh' of the oesophageal ring and the cerebral ganglia, ma\' be, frequent!}', obser\ed in all species. 



Sensory organs. 



In the M}-zostomidae hitherto known, onl}' the cirri, and the tentacles of tlie proboscis, can be considered as spec- 

 ial sensory organs. As alread}- stated, these organs are furnishi-d w itli nerves; from tlie tent.icular ner\'e-ring, one nerve 

 issues to each tentacle; and from the complex nerve-ramification, a bundle of nerves passes into each cirrus. From these 

 nerves, in the cirri as well as, also, in the tentacles, fibrils issue, and connect with the epidermic cells, vide PI. II, fig. 13 and 

 PI. IV, fig. 5. I have been unable to observe the furrow, described In' draft", on the -inferior side of the cirri of M. cirriterum. 

 I have examined man}- transverse, as well as longitudinal, sections of cirri, of M. cirriterum. as also of the other species, but 

 have always found the cirri to be c}-linilrical, nor could I find an}- t^pidermic cells diftering from the others, or ha\-ing an}- 

 resemblance. to .the protrusile glutinous cells ( Klebzellen<) described b\' draft. 



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