20 G- O. Sars. 



delicate inner membrane of the valves. By the rhythmical 

 movements of the branchial lamellæ, and especially of those 

 of the anterior rnaxillæ, a continous renewal of the water 

 takes place within the cavity of the shell, whereby a 

 sufficient aeration of the blood contained between the two 

 lamellæ of the valves may be obtained. 



The nervous system I have not been able to examine 

 more closely, nor could Zencker observe it in its total- 

 ity, owing to its concealed situation, though the upper 

 oesophageal ganglion, and another larger one lying behind 

 the oral aperture were to be faintly traced. 



Of the genital system, only the female organs can be 

 described here, as I have never come across a male speci- 

 men. In all probability males do not exist at all. The 

 ovaria consist of a slender tube on each side contained between 

 the lamellæ of the corresponding valve, and extending 

 immediately above the liver-sac (see fig. 4). This tube 

 gradually tapers distally, and its blind end forms an abrupt 

 curve upwards. At nearly the same place, where the liver- 

 sacs join the intestine, the ovaria pass into the body, 

 forming in the abdominal part on each side a capacious 

 cavity, into which the ripe ova are received. Within the 

 ovarial tubes egg-cells in every stage of development may be 

 observed (see fig. 4). In the hindmost blind ends, a great 

 number of simple nuclei are accumulated. Farther in front, 

 each nucleus has become surrounded with a clear yolk- 

 matter, and the several cells are here more clearly defined, 

 being arranged in several series. By the successively increas- 

 ing size of the cells more anteriorly, only a double series can be 

 discerned, and at last there is only room for a single series 

 of egg-cells successively increasing in size. The cells lying 

 in the foremost part of the ovarial tube become gradually 



