562 8UMMAKY OF CURRENT RESEARCHES RELATING TO 



Testacella.* — Prof. H. do Lacaze-Duthiers has published an in- 

 tercstiug memoir on this Gastroi)od. Altered though its organization 

 may be, and displaced as arc some of its organs, it is still possible to 

 associate it with the rest of the Pulmonata. Though the mantle and shell 

 are very small they both remain as evidence of the parts which are so 

 well developed in allied groups. The only portion of the body which 

 they protect is the true respiratory cavity. 



The details of anatomical peculiarities may be largely cxidained by 

 the drawing down of the mantle and shell, and the elevation of the liver 

 and the organs of reproduction. These two fundamental modifications 

 are the cause of others which are no less important. Thus, when the 

 organ of respiration, which is always intimately connected with the 

 central organ of circulation, changes its place, the heart invariably 

 follows it, and comes to occupy such position as the lung leaves free for 

 it. The same thing happens to the kidney, which is always attached to 

 the pericardium. As a rule the marginal folds of the mantle are quite 

 close to the head, which they often protect, and in consequence of this, 

 the pallial nerves are short. But in Testacella, the mantle is separated 

 from the head, and consequently from the nerve-centres ; the pallial 

 nerves are, therefore, of greater length, though they preserve their fixed 

 relations. Long and delicate nerves, such as those of the foot, float in the 

 general cavity, and are only recognizable by their origins and insertions. 

 " The connections of the nervous system are so constant and imperative, 

 that to follow a nerve is to take in hand the thread of Ariadne which 

 guides and conducts us to the part which it is required to determine, and 

 which, at first, might be misunderstood, in consequence of the trans- 

 formation it has undergone." 



The same is true of the arteries. The heart being removed to the 

 lower part of the body, the organs which have in consequence been dis- 

 placed, have, so to speak, carried the arteries with them. A very 

 interesting relation is presented by the passage across the oesophageal 

 collar of the termination of the ascending aorta. The pedal artery, 

 crossing above the pedal ganglia, ought to pass in front of them to 

 redescend and nourish the foot as far as its lower extremity. This is a 

 constant arrangement in the Pulmonata, but in Testacella, owing to the 

 length of the course which it has to take, the aorta gives off an accessory 

 branch at the middle of its length, which opens freely with the true 

 pedal vessel, and so makes up for the insufficiency of supjdy which is 

 due to the too great length of the latter. Here there is deformation due 

 to elongation, but the relations are fixed, and the parts, modified though 

 they are, have been able to preserve this same relation. 



The superiority of the value of characters which are drawn from con- 

 nections over those furnished by diversity of forms and deviations from 

 the normal is shown by the relative position of the heart and lung in the 

 economy of Testacella. The fixed connection of the two organs is seen 

 in the connection between the auricle and the efferent vessel of the lung, 

 but the relative position of the two, as regards the rest of the body, 

 depends on changes effected in the body in consequence of the displace- 

 ment of some of the viscera. 



Whatever be the cause of the change which it has undergone, we 

 cannot but recognize that Testacella is atrophied in some of its parts and 

 disproi)ortionately developed in others. As compared with a slug, we 



* Arch. Zool. Exper. ct Gen., v. (1887) pp. 459-596 (12 pis.). 



