IX MOLLUSCA 323 



When the macromeres, by further division, have formed the larval 

 stomach, these four cells seem to give rise to the hinder part of the 

 intestine. They may be termed mesendoderm. There is no doubt 

 at all that a similar state of affairs will be found in Patella when the 

 cell-lineage has been fully worked out. It is another proof that the 

 coelomic cells are essentially endodermic in origin. 



The so-called mesectoderm or larval mesoderm, consisting of 

 ectodermal cells which wander inwards and are converted into the 

 muscles of the larval oesophagus, is derived in MoUusca generally 

 from the anterior quadrants (a and b) of the third quartette. 



The stomodaeum in Fiona, Crepidula, and Physa arises from 

 2b'^^^ and 2b^^^ in front, and at the sides from cells of the third 

 quartette, to a certain extent, as in Polygordius. In these MoUusca, 

 however, these third-quartette cells succeed in excluding 2a^^^ and 

 2^222 from the sides of the stomodaeum, whilst behind it is closed 

 entirely by third-quartette cells. In this case 2a^^^ and 2c^^^ give 

 rise to lateral ridges of cells, between which and the opening of the 

 mouth there are grooves filled up by third-quartette cells. In 

 these grooves two pits appear {Physa) which form the rudiment 

 of the radula sac. The ridges formed by 2a^^^ and 20^^^ meet behind 

 the mouth so as to enclose the pits and the mouth in a common 

 atrium or outer stomodaeum ; so that eventually 2a^^2 and 2c^^^ do 

 form the outer stomodaeum. 



The foot in primitive forms, such as Trochus and Patella, is at 

 first double ; it arises from descendants of 2d, in the region of the 

 ciliated groove. 



The most puzzling things about Gastropod MoUusca are the 

 larval kidneys. These appear to be absent in Marine Proso- 

 branchiata, but are found both in Pulmonata such as Zimnaea and in 

 fresh- water Prosobranchiata such as Paludina. In these forms they 

 consist of V-shaped tubes with the apex of the V directed forwards, 

 and they are formed of one huge giant ceU. The internal end, i.e. 

 the upper Umb of the V, is a solenocyte ; the lower limb opens to 

 the exterior not far behind the head and a long way in advance of 

 the opening of the permanent kidneys. At first sight one would 

 be strongly inclined to regard these kidneys as equivalent to the 

 archinephridia of Annelida. But the painstaking analysis of Wier- 

 zejski (1905) has conclusively proved that, in the case of Physa at 

 any rate, the larval kidney arises from three ceUs at the anterior end 

 of a row which is budded from the outer mesodermic teloblast on 

 each side ; and Erlanger (1894) found that in Paludina the larval 

 kidney was segmented off from the anterior tip of the coelomic 

 vesicle on each side. Therefore this type of kidney really belongs to 

 the type which Goodrich terms coelomiduct, and its appearance 

 would seem to indicate that Gastropoda originally possessed two 

 pairs of coelomic kidneys— a conclusion which on other grounds 

 may be regarded as extremely probable. 



In Opisthobranchiata, on the other hand, there is found either 



