REPORT ON THE PYCNOGONIDA. 119 



finally, iu F only two rounded spots of medulla are left {co'), wliicli give off the com- 

 missures between this and the second thoracic ganglion. 



About the histological structure of the ganglia I wish to be very short. In fig. 12 on 

 Plate XYIII. a longitudinal section is given of the first thoracic ganglion of Nijmijhon 

 Irachyrhynchus. A comparatively thick sheet of connective tissue (neurilemma sheath) 

 surrounds the ganglion, and is in continuation with the sheatli of the commissure which 

 unites tliis ganglion with the second thoracic ganglion. Small ganglion cells, each 

 with a distinct nucleus, fill that part of the ganglion wliich is not occupied by the nerve 

 fibres. These ganglion cells are situated in meshes of connective tissue, they are small 

 and furnished with but little protoplasm ; their nuclei are distinct, and show a small and 

 eiitterino; nucleolus. 



In a section through the first thoracic ganglion of Colossendeis leptorhynchus, 

 it is easily observed, that the ganglion cells are of two difi"erent sizes : there are 

 very large ones rich in protoplasm, and furnished with comparatively large nuclei ; 

 there are also very small ones, which show only a small quantity of protoplasm. Fibres 

 and sheets of connective tissue are everywhere observed between the o-ano-lion cells ; the 

 neurilemma sheath of the ganglion itself is very thick, whether it is to l^e considered 

 as really double (an outer and an inner neurilemma sheath, the latter of w^hich should be 

 in connection with the connective tissue meshwork of the interior of the ganglion), I have 

 not been able to ascertain.' 



3. Eyes. — Of the organs of sense I paid si^ecial attention to the eyes. Besides the 

 tactile organs spread over the whole surface of the body, the curiously shaped hairs of the 

 palpi oi Ascorhynchus, and perhaps of other genera, which probably are also organs of a 

 special sense (olfactory ?), the eyes are the only certainly known organs of sense in 

 Pycnogonids.^ In the first place, I tried to ascertain in how far the eyes can really be 

 said to disappear in those animals which inhabit great depths. I therefore drew up the 

 following list, in which the species from the " Challenger " and " Knight Errant " expedi- 

 tions are placed according to the depths they inhabit. 



1 Hithertu I have studied only such sections of the ganglia as are made by cutting the whole body. These are 

 sufficient for researches on the general anatomy, but totally insufficient wth regard to histology. The numerous sheets 

 and strips of connective tissue prevent the substance used for enclosing the body (paraffine or Calberla's substance) from 

 penetrating the ■whole interior of the body. Thus some parts always remain disunited, and the cutting of thin sections 

 is extremely difficult, if not impossible. 



2 The curious organ which Dohm observed between the two eyes on each side of the oculiferous tubercle I have 

 not observed. Perhaps it does not occur in Nymphon, the only genus in which I investigated the eyes. 



