230 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and may be compared to true branehi^. It is noticed, in a description 

 of the main arterial system, that the white appearance of the walls of 

 its vessels is due to the presence of particles of carbonate of lime, and 

 not, as has been supposed, to fatty matter. 



Digestive, Nervous, and Reproductive Organs of OncMdium.* 

 — M. Joyeux-Laffuie discovers, in addition to the radula in the 

 buccal bulb, a chitinous piece, which is of a yellowish-brown colour, 

 is hard and resisting, and is fixed to the anterior end of the base of 

 the bulb ; its surface presents a large number of grooves. It is best 

 seen in large individuals. The oesophagus dilates into a proventri- 

 cular sac ; the gizzard has a chitinous internal membrane, is very 

 rough on its surface, and has the small or third lobe of the liver 

 opening into it. 



The nervous centres are united into a small sub-oesophageal mass, 

 in addition to which there is a small stomato- gastric ganglion. The 

 former is very difficult to dissect out, but with care it is possible to 

 make out in it the following parts : The cerebroid centre is made up 

 of two ganglia united by a long and large supra-oesoj)hageal commis- 

 sure ; from each of these there pass off commissures to the stomato- 

 gastric, pedal, and pallio-visceral centres. The pedal centre is formed 

 by two large ganglia, and is situated behind the pedal gland ; they are 

 united by two commissures, one long and one short, and they give off 

 five nerves to the foot. The pallio-visceral centre is composed of 

 three smaller ganglia, which are united by three short commissures ; 

 the outer ones give off all the pallial nerves, while the intermediate 

 one supplies the circulatory system, the genital organs, and a part 

 of the digestive tube. 



The shortness of the connecting commissures gives rise to the 

 formation of a well-marked triangular body ; from the anterior edge 

 there is the auditory nerve, which passes to the otocyst, placed on the 

 postero-external surface of the pedal ganglion. 



OncMdium is monoecious, but the female orifice is placed behind 

 the anus, while the male orifice is found at the side of the right ten- 

 tacle ; the genital organs occupy the posterior portion of the visceral 

 cavity ; the so-called uterus is irregular in form ; the two albumini- 

 parous glands have their ducts completely distinct, though their 

 constituent lobules are very largely intermixed. The highly "agile" 

 spermatozoa have a dagger-shaped head and a remarkably long tail. 

 The ova, when laid, have a strong shell, and terminate at either end 

 by two j)rolongations, by means of which the eggs are united together. 

 An account of the development is promised. 



Eye of Pecten.t — The general absence of organs of vision 

 amongst the Lameilibranchiata meets interesting exceptions in Pecten 

 and Spondylus, which possess a great number of eyes of considerable 

 complexity situated on the border of the mantle, either at equal dis- 

 tances or clustered together. The number of the eyes also varies in 

 different individuals, ranging in Pecten from eighty to one hundred 



* Comptes Rciidus, xcii. (1881) pp. 144-7. 



t Quart. Journ. Micr. Sei., xx. (1880) pp. 443-55 (2 pis.). 



