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1 have described thcni, lici'c, as l)clon_n;in_L; lo the ncr\'<)us system. In these organs, I lia\'e found two t\'pes of structure, one 

 of them is represented in the structure of M. giganteum, the other in the structure ot M. graft!. In M. giganteum, they con- 

 sist of two gangha rontainiiiL; a large numher of cells, and are situated, one on each side of each parapnihum. 'I'he cells are 

 multipolar, and they each send a prolongation towards the external cxtremit}- of the parapodium. A bundle of fibrils proceeds, 

 in this \va}', from each ganglia, into the parapodium — consequently one on each sidi- — towards the extremity (PI. VI, 

 fig. l). The ner\-e tihres unite, here, with the einthelium round the opening in which the chief hook is situated (PI. VI, fig. 



2 c). In M. graffi, however, the ganglia ha\e quite another structure; in it, the two ganglia of each parapodium are united, 

 and therefore, becomes, a single ganglion onh'. containing but few large cells — 6 — 7 cells — but they are perfect giants. The struc- 

 ture of these gangha, in M. graffi, is illustrati'tl in PI. \'I, fig. 3. In the inferior extremit)', there are, also, a few smaller cells 

 (fig. 3, n., c' ; fig. 10, n., c). The giant cells are, usuall}'. multipolar, and most of their prolongations are directed towards the 

 parapodium The prolongations are distinctl}' fibrillous; I ha\'e not observed any histological difference between them. The 

 nuclei are large, and oval, with distinct nucleoli (fig. 3, b., fig. 4, b., fig. 11; PI. \'II, fig. 12, 13, 14, 15). A peculiar organ is 

 situated at the inferior extremity of the ganglion, having the form of a glass bulb-receiver with the long receiver-neck 

 passing towards the extremit\- of the parapodium. The globular, upper, extremit}- of this organ has the structure illustrated 

 in PI. \'II, fig. 1—7; it consists of several external concentric la)-ers, and an inner, apparentl}- coagulated, substance, vide fig. 

 2 and 7. This bulb communicates with bundles of fibres issuing from the cells (PI. M. fig. 3 and 12, PI. VIII, fig. I, 4, 6, 7). 

 The prolongation is hollow, and contains a substance resembling that of the bulb (PL VI, fig. 8, xu.). I ha\-e been unable to 

 determine how this prolongation terminates, or whether it is hollow, quite to its extremity, or not (PI. \'II, fig. 9, xu.). An\- 

 nucleus belonging to this organ, is not, with certainty, observed. PI. Ml, fig. 3, a. illustrates a nucleus that can be seen situated 

 in the uppermost extremity; it is surrounded by a vacuolar protoplasmic substance, and I think that it belongs to the 

 organ. PI. MI, fig. 5, a. illustrates a similar nucleus that may be seen, rituated in a constrictive cavity communicating with 

 the bulb. I have, occassionally, found nuclei, situated in a similar manner. These nuclei belonged, probably, originally, to cells 

 from which these organs, by a process of transformation, hax'e sprung, whilst the nuclei receded more and more, into constric- 

 tive cavities (.^^t Each of these ganglia, in M. graffi, communicate with the \-entral cord by 4 nerves, vide PI. \T. fig. 5 

 n', n-, n' and n*. The nerves n' and n- are analogous to those found in M. giganteum. Ner\'cs analogous to n-' and n-^, I 

 have not obser\'ed, here, with certaint}'. 



Histologi/ of the nervous system. 



The neurilemma-sheath of the nerves and the ventral cord, consists of an outer, and an inner sheath. 



The outer sheath consists of a stout, homogeneous membrane, which is \'cr\- prominent in transverse sections, 

 owing to its deep staining, and sharp outlines, of which the inni.'r one is the sharpest. This memlirane contains no 

 nuclei, Init I have, however, found man)' such adherent to the exterior side. I think it must be regarded as a cuticu- 

 lum, derived from the layers of connective-tissue situated on its outer side, vide PI. \', fig. 12. The homogeneous 

 structure of this membrane appears to vary a little because I have found, in M. graffi, especially in its tlorsal portion, 

 parts that stain difterentl}', and some of them somewhat granular, vide PI. V, fig. lO and PI. IX, fig. 2 and 3. Septa issue 

 from the homogeneous meml.irane, into the inner nt-urilemma sheath and inner parts of the ventral nerve-cord. The inner 

 neurilemma-sheath fills the spaces between the ganglionic cells, the fibrillar cords, and the outer homogeneous membrane; 

 it is constructed of layers of connective-tissue which form a reticulation, and provide the fibrillar cords, as well as the gang- 

 lionic cells with membranes. These cellular memliranes are continued into, and surround the prolongations. The connective- 

 tissue, forms thus, sacks in which the cells are situated. Abundance of nuclei occur in the connective-tissue, situated, iisuallj', 

 in the fibres which form the meshes, and not in the middle of the meshes themselves. Small septa pass into the 

 fibrillar mass proceeding from the membranes enclosing the longitudinal commissures. In tliis mass, connective-tissue 

 nuclei occur sparing!}', and usually, only in conjunction with such small septa; the}' can, I'speciall}". be observed where large 

 nerves are gi\'en oft', vide PI. \", fig. 16. The outer, as well as the inner, neurilemma-sheath is continued into tiie peripheric 

 nerves, which, also, in the same manner, are provided with double sheaths, as descrilied b}' Hermann in Ilirudo. B}' means 

 of the septa of the inner neurilemma-sheath, the nerves are subdivided into a great man)' tubes; in these septa, connecti\'e- 

 tissue nuclei occur. The cerebral ganglia are, as above mentioned, unprovided with an\' special neurilemma-sheath. The cells 

 have, however, connective-tissue nn'iiibranes. The surrounding coiinecti\e-tissue, also, shows a tendenc)' to difierentiation, 

 as it has, especialh' in certain species, a somewhat lamellar structure. In M. graffi. I h.ive observed that the cells sur- 

 rounding the tentacular nerve-ring are situated in vacuoli, ;ind are thus, provided, with a double memljrane, vide PI. I\', 

 fig. 6, b. aiul a. I ha\'e not observetl muscular fibres t-mbedded in the neurilemma-sheath. but ha\e, however, observed 

 muscles close to the exterior sides. These muscles are, I believe, secured to the outer sheath, by one of their extremities, 

 whilst the opposite cxtremit)' terminates in the surrounding connective-tissue. 



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