212 Description of the Plates. 



placed superiorly, and vesicular placed inferiorly. The latter 

 form a continuous stratum with ag-greg-ations at intervals 

 corresponding to the middle of each segment, which are 

 called ganglia, and give off two pairs of nerves on either 

 side. The nerve-cord gives off in each segment anteriorly 

 to the two pairs arising from the ganglioniform intumes- 

 cence a single pair, which distributes itself along the line 

 of the anterior dissepiment of each segment, and appears to 

 correspond to the Nervi transversi of the Arthropoda. The 

 four nerves given off in the middle of each segment are 

 accompanied by branches from the pseud-haemal vessels 

 which run on either side of the nerve-cord; the two nerves 

 given off anteriorly to them are similarly accompanied by 

 branches from the azygos pseud-haemal vessel which under- 

 lies the nerve-cord. These nervous and vascular branches 

 are not given in this Plate. 



b. Pharynx, turned aside to the left, the right half of the organ, 

 except the small portion upon which the right stomato- 

 gastric plexus, a %, is seen, having been removed. The 

 walls of the pharynx are of great thickness superiorly, 

 glandular tissue forming the exterior, and muscular the 

 middle layers, whilst the mucous membrane forms the inner- 

 most, and attains a considerable thickness where it lines the 

 saucer- or sucker-shaped cavity which opens from above into 

 the cavity of the tube. 



CI. First muciparous gland, or ^segmental organ,' opening 

 externally in segment iv. Ordinarily the thickened mus- 

 cular portion of the tube of the convolutions of which these 

 glands are made, opens externally in the segment imme- 

 diately posterior to that in which its internal funnel- 

 shaped opening is situated, this internal funnel-shaped 

 opening being carried upon a short hollow stalk prolonged 

 through the anterior muscular dissepiment of each posterior 

 segment of the two with which each segmental organ is 

 connected. But as segment iv. is not limited off from seg- 

 ment iii. by any such perfect or nearly perfect dissepiment 

 as limits the segments after segment v. from the seg- 

 ments next in front of them, the anterior funnel-shaped 

 termination is not seen so distinctly to be in a different 



