THE RISE OF THE LARVA, ETC. 123 



has been assumed or conjectured in connection with other forms (Annelida, 

 Echinodermata) and specially for the Mollusca (cf. Cyclas, p. 29). It must be 

 regarded as a striking fact that even those zoologists who, like EELANGER, 

 are very decided as to the derivation of the whole mesoderm from the meso- 

 derm-bands, allow that some of the elements of the connective tissue arise 

 from the ectoderm. For example, the so-called " nuchal " cells on the pos- 

 terior edge of the velum on the " neck," i.e., an accumulation of specially 

 large ectodermal cells, pass inward so as to become distributed in the con- 

 nective tissue. Although of different appearance from the other elements of 

 the connective tissue, they appear to belong to the latter.* 



4. The Rise of the Larva and its Relation to the Adult Form. 



The variations which we have found in the development of the 

 germ-layers among the Gastropoda naturally lead us to expect varia- 

 tions in the external form of the embryo. In the development of the 

 latter, an important part is played by the smaller or larger amount 

 of yolk contained in the egg. Besides this, however, adaptation to 

 the manner of life of the various forms has to be considered, for the 

 greater number of Gastropod larvae swim about freely for a long 

 -time before assuming the adult form. Now although the larvae, in 

 essential points, can be traced back to a fundamental form, the 

 differentiations found in the various divisions are somewhat far- 

 reaching, so that we are obliged to consider the different larval forms 

 apart. We shall first, however, describe the development of a few 

 specially characteristic forms so as to give the reader a general 

 idea of the subject and to make possible a comparison with other 

 divisions of the Mollusca. 



The development of the larval form of Patella has been described 

 in detail by PATTEN (No. 83), and since this Prosobranch, which 

 belongs to one of the most lowly groups, apparently most nearly 

 attains the typical larval form, its ontogeny will occupy us first. 

 Unfortunately the development of this form has only been followed 

 by PATTEN up to a stage at which the larva is still far removed from 

 the shape of the adult. 



The ontogeny of Patella shows primitive conditions in so far as 

 the egg-envelope is thrown off very early, even while cleavage is still 



* [The above somewhat conflicting accounts of the rise of the mesoderm, 

 taken in connection with the more recent observations of CONKLIN (No. IV.), 

 HEYMONS (No. XII.), and WIERZEJSKI (No. XXVII.), seem to render it highly 

 probable that the middle germ-layer has, in all Gastropoda, as has been 

 suggested for the Lamellibranchia, a double origin : (1) from primitive 

 mesoderm-cells giving origin to the lateral mesoderm-bands ; and (2) from 

 the ectoderm at a later stage as paired differentiations nearer the anterior 

 end of the body. ED.] 



