310 INVEETEBEATA CHAP. 



has it been done satisfactorily in the case of any Mollusc except 

 Paludina. 



Paludina is, like Patella, a univalve or Gastropod, but it differs 

 from Patella in possessing a spirally coiled shell. It is a fresh- water 

 form found abundantly on both sides of the Atlantic, Its peculiarity 

 is that the lower part of the oviduct is enlarged to form a kind of 

 womb within which the eggs undergo their complete development, 

 leaving the body of the mother only when they have attained, 

 externally at least, a complete likeness to the adult. If a number of 

 adults, then, be collected and killed in an extended condition, by the 

 slow addition of minute quantities of chloral or cocaine to the water 

 in which they are living, and if the shells be then carefully picked 

 away piece by piece and the oviducts slit open, the contained embryos 

 may be washed out by aid of a pipette into a watch glass of normal 

 salt solution, examined fresh, and afterwards preserved by the 

 corrosive sublimate and acetic acid mixture. 



The egg is very minute ; it divides regularly into blastomeres of 

 nearly equal size, the niicrorneres being as big as the macromeres ; it 

 forms a regular trochophore and then a veliger; in fact it pursues 

 a primitive development within the oviduct, by the secretion of which 

 it is nourished, and it does not depend for sustenance on the yolk 

 contained in its own cells. 



We owe to Erlanger (1891, 1892) an exhaustive account of the 

 development of Paludina ; and his results, so far as the later stages are 

 concerned, have been confirmed and extended by Miss Drummond 

 (1902). So far as the earliest stages are concerned, however, Tonniges 

 (1896) has directly contradicted Erlanger's statements; and on this 

 controversy a few general remarks may be made. 



Erlanger's account, written before the days when cell-lineage was 

 studied, commences with the gastrula stage. This stage is reached- 



by a process of regular invagination such as 

 is found in Polygordius, not by a massive 

 inflow of large cells as in Patella. The 

 blastopore, according to him, persists as the 

 anus, and the stomodaeum is formed in front 

 of it, so that the mouth is a new perforation. 

 The prototroch appears, as in Patella, as a 

 double circle of cilia carried by two rows of 

 cells. On the ventral side of the intestine a 

 median bilobed pouch is formed, which is the 

 , origin of the mesoderm (Fig. 245). This pouch 



FIG. 244. Vertical section of J ., 



the gastrula of Paludina becomes cut on trom the gut, loses its cavity, 

 vivipara. (Aftur Brian- and gives rise to two irregular mesodernial 



)O O 



bands which extend forwards at the sides of 



'' A 1>o1 the gut. Each of these bands gives off a 



small compact mass at its anterior end (l.n, Fig. 246), which becomes 

 converted into a larval kidney (Erlanger, 1894); while the rest 

 of the streak breaks up into an irregular mass of stellate cells 



