Introduction to A7iiinal MorpJiology, 265 



labial, pallial and anterior branchial branches. The 

 hypopharyngeal ganglion is united to the former bv two 

 commissures, one on each side of the digestive canal. 



It is sometimes close to the first ganglion (Pecten), but is 

 usually remote, placed in the foot, hence it is called the pedal 

 ganglion (or, from its describer, the Mangilian). When the 

 foot is small or absent, the ganglion is small or obsolete, as 

 in the oyster ; it is also small in Teredo. It sends branches 

 to the retractors of the foot, to the adductors, to the body 

 wall, but none to the viscera. From the epipharyngeal gan- 

 glion two lateral commissures pass backwards to the front of 

 the hinder adductor, where they unite to form a visceral or 

 anal ganglion, giving off a posterior branchial, and lateral 

 and posterior pallial branches (the first of these branches in 

 Ungulina has a separate ganglion on it at its origin). These 

 ganglia are usually brightyellow, translucent, and contain round 

 fat cells, small, colorless, medullary cells, and non-meduilated 

 nerve fibres. They are symmetrically disposed in cquivalve 

 forms, but are unsymmetrical in inequivalves. A few fila- 

 ments from the epipharyngeal ganglion represent the labial 

 nerves of Brachiopods, In Unifora there are two long circum- 

 pallial nerves along the margins of the mantle lobes ; some- 

 times there are small ganglia along this line in Trifora, as in 

 Solen. 



As organs of sense, there are, ist, labial tentacles; 

 2nd, marginal tentacles, supplied by the circumpallial 

 nerve ; thread-like in Lima, Pecten, &c. ; thicker and 

 contractile in Donax and Mactra, sometimes highly 

 extensile (Lepton). In siphonate forms these are 

 grouped about the mouth of the siphon. In Unio 

 there are a few papillary tentacles at the hinder end 

 of the mantle border. A siphonal ganglion supplies 

 these in the siphonate forms ; 3rd, otocysts exist, one 

 on each pedal ganglion ; they are oval, with a homo- 



