124 HANCOCK AND EMBLETON OF 



brancliijB. Third, tlie branchial circulatioiij in which flows onlj 

 the more deteriorated blood, brought by the hepatic vein, but in 

 which also that blood undergoes the highest degree of purification, 

 capable of being effected in the economy, namely in the special 

 organ of respiration. 



This triple circulation has not, so far as we are aware, been 

 heretofore described as existing in the Molluscan Sub-Kingdom. 

 From the fact of the blood in Doris being returned to the heart 

 in a state of partial aeration, it is clear that this animal is on a par 

 with the higher crustaceans, and from the blood arriving at the, 

 heart in the same condition, according to the researches of Garner 

 and Milne Edwards into Astrea and Pinna, the great Triton of 

 the Mediterranean, Haliotis, Patella, and Helix, it can scarcely 

 be doubted that this arrangement will be found throughout the 

 molbisca. In the brachiopods, according to the observations of 

 Professor Owen, the blood must be assumed to be returned to the 

 heart in the same condition, since no special respiratory organ 

 exists in these animals, with the exception of Lingula, in which it 

 is in a rudimentary state. From a consideration of the facts cited 

 it may be deduced that the skin or mantle is, iu the mollusca, 

 the fundamental organ of respiration, and that a portion of that 

 envelope becomes evolved into a speciality, as we trace upwards 

 the development of the respiratory powers. 



Organs of inney^vation. — These are in two divisions; one cor- 

 responding to the cerebro-spinal system, the other to the sym- 

 pathetic, or ganglionic, system, of the vertebrata. The existence of 

 the latter is now, for the first time, fully established. The first 

 system is made up of seven pairs of ganglia, and a single one, with 

 fifteen pairs of nerves, and four single nerves. The pairs of ganglia 

 are symmetrically placed with regard to each other, and to the 

 median line. The single ganglion, not before described, has been 

 named visceral. Of the seven pairs, five are suj^ra-oesophageal, 

 two infra-oesophageal ; the single ganglion is attached to the under 

 surface of the supra-oesophageal ganglia of the right side. 



Of the supra-oesophageal ganglia three are large; the anterior 

 pair, the cerebroid, supports the olfactory and the optic ganglia, and 

 also the auditory capsule. The cerebroids supply with nerves the 



