376 Miscellaneous. 



The velum occurs in all the aquatic Pulmonata, iu which, however, 

 it is but slightly developed, and occurs only in the form of a zone of 

 cilia, interrupted on the back, and extending from the mouth to the 

 dorsal vesicle. In Helix the velum affects the same form, and forms 

 two ciliated crescentiform ridges, which extend from the mouth to 

 the neighbourhood of the shell-depression. 



The primitive kidney, which had previously been observed only 

 in the terrestrial Pulmonata, occurs also in all tbe aquatic Pulmonata. 

 In its origin it is a depression of the ectoderm, which is formed im- 

 mediately below the velar ridge on each side at its posterior third, 

 and becomes elongated forwards. The anterior part is not glandular 

 in the aquatic Pulmonata ; it presents the form of a ciliated tube, 

 which oj)ens like a funnel in the interior of the body a little above 

 the mouth. It consequently affects the same form as the segmental 

 organs of certain Vermes. It is this organ that has been taken in 

 Limnmis, by M. Rahl, for the oesophageal ganglia. It is undoubtedly 

 also this organ that M. Ganine saw, but describes as a pair of large 

 colls furnished with long efferent ducts. 



A little above the vibratile funnels of the primitive kidney, a 

 mass of cells is seen to become detached from the ectoderm. These 

 cells, which Mr. E. Pay Lankester has eiToneously taken for the 

 origin of the cerebroid ganglia in Limnasus, in reality only give 

 origin to conjunctive tissue. The cerebroid ganglia are subsequently 

 formed, at the moment when the tentacles begin to push forth ; 

 they detach themselves from the ectoderm at the base of the anterior 

 side of the tentacles within the zone of the velum ; the process by 

 which they are detached is a simple folding in Ancylus and Planorbis, 

 but a well-marked invagination in the terrestrial Pulmonata. The 

 eyes appear at the upper part of the tentacles, and the otocysts at 

 the sides of the base of the foot, by the same processes of formatior 

 as the cerebroid ganglia. The pedal ganglia are always separatee 

 from the ectoderm of the sides of the foot by simple folding. 



The foot of the aquatic Pulmonata contracts alternately with the 

 neck, thus producing a larval circulation. In the terrestrial Pulmo- 

 nata the extremity of the foot becomes converted into a great con- 

 tractile vesicle, which contracts alternately with the dorsal vesicle 

 This pedal sinus in Avion has the form of a very long gut ; it 

 Limax and Helix it is broad and flattened; and in Helix pomatia il 

 attains such dimensions that it lines the whole inner surface of tht 

 egg-shell. We also find, in Helix, on the right side a true larval 

 heart like that of the Prosobranchiata. This larval heart afterwards 

 passes into the pallial cavity, and does not cease beating until long 

 after the definitive heart is formed. The definitive kidney is formed 

 as in the Pteropoda, and communicates with the cavity of the peri- 

 cardium by a ciliated duct. The heart appears as a simple contrac- 

 tile cavity in the midst of the mesoderm, and afterwards surround? 

 itself with a pericardium. 



Thus the type of development of the Pulmonate Gasteropoda 

 differs but little from that of the freshwater Prosobranchiata, which 

 I have also studied. — Comptes llendus, September 27, 1875, p. 523. 



