DEVELOPMENT OF Till. EXTERNAL FORM — PUL.MONATA. 179 



externally. The apertures of the two kidneys lie at the two sides of 

 and behind the velum. The inner end of the primitive kidney is 

 usually regarded by authors as communicating with the primary 

 body-cavity by a ciliated aperture.* In the terrestrial Pulmonates 

 this has been maintained with certainty for Helix (Acavus) by 

 P. and F. Sarasin (No. 10l>) and Jourdain, as well as Meuron 

 (Nos. 50 and 75), arrived at the same result. 



The primitive kidneys of the terrestrial Pulmonates, which were 

 early recognised by O. Schmidt and Gegenbaur, are somewhat 

 differently constituted from those of the aquatic forms. They also 

 ha\c the form of bent tubes opening externally through wide aper- 

 tures in front of the border of the mantle, but they are composed 

 of a large number of cells arranged like an epithelium, none of which 

 are distinguished by their special size (Jourdain, Meuron, Sarasin). 



De Meuron considers that, in Helix, the primitive kidney arises chiefly 

 from the ectoderm, but holds also that the innermost part may be derived 

 lrom the large mesoderm-cells. But since these latter, in the aquatic Pul- 

 monates, yield the principal part of the primitive kidneys, the derivation of 

 these organs from the mesoderm appears more probable. We need not, 

 however, exclude the supposition that, as in the primitive kidneys of the 

 Prosobranchs, an ectodermal invagination takes part in the formation of 

 the peripheral part and that this latter, in terrestrial Pulmonates, is specially 

 extensive. 



At the time when the primitive kidneys attain their full develop- 

 ment, the external form of the embryo also undergoes further altera- 

 tion. The shell-gland begins to lose its pouch-like form and gradually 

 flattens out. The ectodermal epithelium belonging to the shell-area 

 still appears formed of columnar cells. Over this area lies the shell 



* [v. Erlanger (No. VII.) has since described the detailed structure of the 

 larval kidney in Planorbis and Limnaea ; he finds a specialised ciliated cell 

 (the funnel-cell) which puts the tube into communication with the body-cavity, 

 and then a long tubular segment containing a flagellum and a terminal por- 

 tion which opens on to the exterior, this latter portion Erlanger thinks may 

 be ectodermal in the Euthyneura, while the remainder is mesodermal. In 

 the Pulmonata he finds a swollen ampulla at the junction of the two seg- 

 ments. The development of this organ has been more recently investigated 

 by Meissenheimer (No. XVII.), and this observer maintains that, in Limax, 

 the primitive kidney is wholly ectodermal, aud here he is at variance with most 

 other observers. As he also maintains that the heart and definitive kidney 

 similarly arise from a common ectodermal rudiment, we think that his views 

 require further confirmation before we can accept them. Meissenheimer (No. 

 XVIII.) has also given a most elaborate account of the structure of this organ 

 in which he differs from Pan. anger in one important respect, viz., he is un- 

 able to find any opening into the body-cavity and thinks that Erlanger 

 mistook a large vacuole which is invariably present in the end-cell for an 

 opening. — Ed.] 



