118 THE VOYAGE OF H.M.S. CHALLENGEE. 



distinct nerves are observed arising at an acute angle from the commissures. These 

 innervate the muscles whicli run from the one segment to the following one. 



I give a figure (Plate XVIII. fig. 10) of one of the smaU ganglia and the nerve 

 fibres arisins; from it on the inner surface of the integument. I observed these ganglia 



O O DO 



in Nymphon and in Colossendeis ; they are more numerous in the latter genus, and 

 especially numerous and in a better state of preservation in one of the specimens of 

 Colossendeis j^^'oboscidea, Sab., dredged last summer north of Scotland. The figure is 

 taken from a part of the integument of a specimen of this species. 



To show the minute structure of the ganglia I give in fig. 1 1 A-F a series of six figures 

 illustrating vertical (frontal) sections through the supra-oesophageal and first thoracic 

 ganglia. As will be easily seen, the sections do not form a complete series, they are only 

 the most interesting out of a series of about twenty. Fig. A represents a section 

 c[uite at the front of the supra-oesophageal ganglion ; a distinct and comparatively thick 

 neurilemma sheath surrounds the whole ganglion, and three distinct medullary nuclei 

 (pointed sul^stance according to Leydig) show the place of origin of the tliree main 

 nerves. The rest of the section is composed of ganglion cells, with the exception of a 

 small meduUary spot at the left side, which does not occur at the other side, 

 and proves that the section is somewhat oblique. Neither the oesophageal com- 

 missures nor the fii'st thoracic ganglion are yet to be seen in this section. Fig. 

 B represents a section which passes through the supra-oesophageal ganglion and 

 through the foremost part of the first thoracic ganglion, but not yet through the 

 commissures. Almost the w-hole section is occupied by the medulla, which forms 

 regular prominences towards the periphery, and only a very small space is occupied l)y 

 the ganglion cells ; m is the lobe for the mandibular (antennary) nerve, o represents the 

 lobe for the optic nerve, and x is a median lobe about the function of which I do not feel 

 (piite sure. In section C this median lobe is no longer to be observed, and the optic 

 lobes have here assumed a much more elongated form. In section B the commissures 

 are still totally wanting, but the front part of the first thoracic ganglion, with the 

 meduUary nuclei (j)) for the two infra-proboscideal nerves, are distinct. These in 

 section C are reduced to small lobes, while those for the nerves of the ovigerous legs (t) 

 are distinct. In this section the oesophageal commissures are seen, and also the under- 

 most parts of the two optic nerves, which arise from the supra-oesophageal ganglion 

 (o'). Their connection with the optic lobes, however, does not occur in this section. 

 Section C passes almost through the middle of the oculiferous tubercle, so that two eyes 

 are placed in front of this section and the two others behind it. Section D represents the 

 last part of the supra-oesophageal ganglion ; the hindmost part of the medulla is seen, and 

 the ganglion cells begin again to increase in number. Of the first thoracic ganglion, the 

 section goes through that part of the medullary centrum which gives off the nerves fur 

 the ovigerous legs (/). In E this medullary centrum is considerably less voluminous, and. 



