134 GASTROPODA. 



that it may have been derived from a marine ancestor (possibly an 

 Opisthobranch). Onchidium lives in the littoral zone, within the reach 

 of the tides, hidden in rocky fissures, where it lays its gelatinous 

 egg-masses. These are washed by the sea water, and JOYEUX- 

 LAFFUIE was able to develop them by keeping them damp and 

 immersing them from time to time in sea water. The eggs therefore 

 develop under conditions not very different from those of marine 

 Gastropods. 



Although, as a rule, the Veliger stage is much reduced in fresh- 

 water and terrestrial Gastropods, the Trochophore form is still more 

 or less distinctly developed in them. In the Pulmonates, the 

 Trochophore stage is present but is not very conspicuous (p. 177) ; 

 in Paludina, however, it is unmistakable, although this Gastropod 

 is viviparous (Fig. 56). Paludina, in many other respects besides 

 the retention of the Trochophore stage, is an archaic form and,. as its 

 ontogeny has been so carefully studied, we shall give a special 

 account of its development. The principal sources of our knowledge 

 concerning the ontogeny of Paludina are the works of LEYDIG (No. 

 68), RAY LANKESTER (No. 64), BUTSCHLI (No. 18) and v. ERLANGER 

 (No. 27), and we have also observations made by RABL (No. 92) and 

 BLOCHMANN (No. 8). ERLANGER' s account is the most recent and 

 the most complete in every respect. 



The Development of Paludina. The fertilised egg of Paludina 

 vivipara develops into an almost spherical blastula which becomes 

 somewhat flattened later and contains a distinct cleavage-cavity. 

 The flattening takes place in connection with gastrulation, the 

 cleavage-cavity during this latter process being almost completely 

 obliterated by the development of the archenteron, so that a stage 

 is here brought about similar to that which occurs in other Gastro- 

 pods, especially in Firoloida (Fig. 44 B, p. 114). The gastrula, 

 which at first is almost kidney-shaped, with a wide blastopore, 

 expands by growth and becomes bell-shaped in Firoloida (Fig. 44 C). 

 The blastopore narrows to a slit. 



The formation of the mesoderm has already been described. On 

 this point, we follow the older statements of BUTSCHLI, according to 

 which the mesoderm is present in the form of two mesoderm- bands 

 which, during the gastrula- stage, consist of few cells but increase 

 later (Fig. 56 A), i.e., show the same condition as in other Gastropods 

 (cf. p. 121). The greater part of these bands soon undergoes dis- 

 integration, breaking up into separate cells which become irregularly 

 distributed in the cleavage-cavity. 



