DESCRIPTION OF PLATES. 821 



exposing the peristomial and peri-oesophageal haemocoels and the diaphragm which separates 

 them from one another behind the cerebral capsule. 



a.i. Dorso-posterior portion of ala infundibuli. 



ao.s. Aorta superior. 



a.p.a. Anterior proventricular artery. 



b.c. Buccal cone. 



c.c. Cerebral capsule attached to neighbouring structures by conjunctive trabeculae and 

 produced in front as a broad lobe. 



er. Crop. 



ni.i. Inferior mandible. 



n.m. Nuchal membrane. 



pr.m. Protractor muscle of the buccal cone. 



r.i.m. and r.s.rn. The dorsal retractor muscles of the buccal cone, r.i.m. attached to 

 the inferior, r.s.m. to the superior mandible. 



N.B. The cerebral commissure only occupies the posterior band-like portion of the massive 

 fibrous capsule. It is possible that this fibrous capsule represents a primordial condition of 

 the cartilaginous capsule of Dibranchs which is not, I think, homologous with the capito- 

 pedal cartilage of Nautilus, the latter being essentially a funnel (siphonal) cartilage and not 

 a cranial cartilage. 



PLATE LXXXII. 



Pig. 1. N. macromphalus $ juv. Innervation of the infrabuccal organ (organ of Van der 

 Hoeven). The nerve to each lamelligerous lobe (i.e. to each half of the organ) ends abruptly, 

 giving off the lamellar nerves as shown on the right side only (left of the figure). 



m. Shallow secondary fold of the buccal membrane which would form the adoral portion 

 of the sheath of the organ. 



The middle length of the hood of the specimen was 52 mm. ; the longer diameter of 

 each lobe of the infrabuccal organ measured 5 mm. 



Fig. 2. N. pompilius £ juv. Innervation of infrabuccal organ ascertained from an injected 

 specimen. 



a, b and c. Branches of the infrabuccal artery which accompanies the nerve. 

 Middle length of hood 65-5 mm. ; length of infrabuccal organ 6 mm. 



Fig. 3. Topography of the infrabuccal organ of the male. The fore-part of the funnel 

 has been cut away and the ventral symphysis divided down the middle, thus exposing the 

 ventral portion of the peristomial haemocoel, the organs which project into it and the nerves 

 and vessels which traverse it. The dotted line represents the limit of the fundus of the 

 external fossa buccalis. 



p.c. Capsule of the coronal (pleuro-pedal) commissure, from which a small median artery 

 issues and, passing freely across the haemocoel, enters the fundus of the infrabuccal organ. 



n. Infrabuccal nerve accompanied by the artery. 



l.i. M. levator infundibuli. 



c.i. Cartilage. 



a.i. Ala infundibuli. 



Fig. 4. Somewhat similar dissection of female (JV. macromphalus). The fundus of the 

 peristomial haemocoel is exposed from the ventral aspect. 



107—2 



