278 DE. H. J. FLEUEE ON THE EVOLUTION OF 



ultimate disappearance of the left cteuidium. In Acmcea fragilis. Miss Willcox found 

 a muscular strengthening of the excurreut channel of the large right kidney and 

 descrihed it as a penis, a description which raises special difficulties and is highly 

 improbable. In the light of the above considerations, it is, however, easy to explain the 

 development of muscularity as an attempt to force excreta beyond the limits of the gill. 



Haller has shown that, even before the loss of the left ctenidium, the development of 

 respiratory outgrowths occurs along the ventral surface of the mantle-skirt. With the 

 loss of ctenidial respiration among the Cyclobranchs, this process is carried much further 

 and a circle of gills appears on the mantle-skirt which — in Ancistroinesus, for example — 

 are even branched. 



Tlie roof of the branchial cavity in Cyclobranchs is thin and honeycombed with blood- 

 spaces, so that it probably serves for the breathing of damp air among those forms which 

 live high up the shore. 



With the loss of the right ctenidium, the right auricle of the heart was reduced, 

 and variations in the position of the heart were selected which did away with the 

 now purposeless curves in the blood-stream. This involved the shifting of the heart 

 towards the left, so that tlie auricle lay directly behind the remaining ctenidium. 

 The ventricle would then turn to the left, so as to lie behind the auricle, and its 

 previously antero-posterior axis would run obliquely backwards from right to left 

 (PI. 16. fig. 16 a-c). The communication between ventricle and aorta shifted at the same 

 time to a position directly behind the auriculo-ventricular aperture. The pressure of the 

 consolidating viscera meanwhile reduced the size of the pericardium, and the presence of 

 the rectum became rather an obstruction than a support, so that we find complete 

 separation of the two, the rectum running parallel to the ventricle, but just outside tiie 

 pericardium. 



A feature of the Docoglossan heart is that the ventricle is united to the dorsal wall of 

 the pericardiuna along its morphological longitudinal axis, i. e. along a line going 

 obliquely backwards from right to left (figs. 17 & 18, Meso.). It is accepted that the 

 pericardium arises as a pair of sacs, one on each side of the rectum, and that the 

 ventricle of the lieart develops in the musculature of the partition-wall between them, 

 either above, around, or below the rectum. The remains of the partition are " meso- 

 cardia," bvit they usually disappear in Gastropods. The connection between ventricle 

 and pericardial wall in Fatella may be such a inesocardium, possibly a secondarily 

 persistent embryonic character : figs. 18 a and b show this condition as seen in section of 

 Acmcea and of Patella. A mesocardium occurs in Chiton, and it is quite possible that 

 it was also present in the Prostreptoneure. 



The Docoglossa possess a special intrapericardial " Bulbus Aortse " sej)arated from the 

 ventricle by a valvular aperture. It is a development of the base of the aorta, and 

 is relatively strongly muscular, esjiecially on the side against the ventricle. Its special- 

 ization is probably due to the fact that the two arterial streams diverge in opposite 

 directions, both of which are at 90° to the direction of the blood-stream through the 

 ventricle. This curve could not be straightened out as the others had been, and, as an 

 alternative, extra muscular tissue has been developed at the basis of the aorta which is 

 included within the pericardial cavity. 



