DEVELOPMENT OF THE EXTERNAL FORM — HETEROPODA. 153 



parasitic Gastropods show decided changes in their structure, and 

 this would be still more the ease if they were to penetrate through 

 the integument of the host and reach the body-cavity. The possibility 

 of such a breaking in of the parasite from without is shown by the 

 StiliftH', which has already buried itself deep in the skin. The 

 cMerual shape as well as the inner organisation finally undergo, as 

 in many other parasites, such a far-reaching alteration, that there is 

 hardly any resemblance left to the former Gastropod, the parasite 

 having degenerated into a mere tube, like Entocolax or Entoconcha, on 

 which are devolved the functions of feeding and reproduction alone 

 {W. Voigt, Xo. 129; Braun, No. 15; Schiemenz, No. 108). 



B. Heteropoda/ 



The ontogeny of the Heteropoda closely resembles that of the 

 Prosobranchia to which in other respects also they are nearly related, 

 but the special form of the adult Heteropod determines certain 

 variations especially affecting the later stages of development. The 

 ontogeny of the Heteropoda has been made the subject of special 

 study by Leuckart (No. 67), Gegenbaur (No. 37), Krohn (No. 

 58a) and Fol (No. 31). 



We have already become acquainted with a few of the younger 



stages of the embryo of Firoloida (Fig. 44 



A-O, \>. 1 14). The oldest of these stages 



was an invagination-gastrula. The inner 



end of the archenteron soon assumes a 



remarkable bilobed form, which recalls the 



enterocoelic formation of the mesoderm as 



described by Erlanger in connection with 



Palvdina (p. 121), but which is no doubt 



explained by the fact that the shell-gland 



. . a Fig. 64.— Embryo of Fvroloida 



which arises dorsally grows as a conical Desmaresti (after Fol). c, 



• j . , i the primary body-cavity ; q, 



invagination towards the archenteron, archenteric cavity ; o, mouth ; 



causing a depression in the latter. When £• f°°* ; s> shell-gland ; s\ 



1 shell-plug ; v, velum. 



the shell-gland begins to flatten out again 



(Fig. 64), the archenteron also assumes a more regular form, becoming 



wider and sac-like. The blastopore passes over into the permanent 



mouth (Fig. 64, >>). The shell-gland at first appears tilled by a plug 



of brownish substance (s') ; in Palvdina, where a similar feature was 



* [The Heteropoda, or Nucleobranchia, are very generally regarded as a 

 minor branch of the Prosobranchia, being classed under the Monotocardia as 

 a subdivision of the Taenioglossa.— Ed.] 



