142 GASTROPODA. 



cases have become known of certain relations existing between the 

 anus and the blastopore. v. ERLANGER, for instance, described the 

 blastopore of Bythinia as a slit, the posterior end of which lies at the 

 spot where the anus forms later (Fig. 96 B, p. 210), and even in 

 Paludina itself it appears indisputable that the slit-like blastopore 

 extends almost to the velum, i.e., to the spot which corresponds to 

 the mouth that forms at a later stage. Further support for this view 

 is found in the condition of some Opisthobranchs (Doris, Aplysia, 

 according to LANGERHANS and BLOCHMANN), in which the anal cells 

 are found exactly at the posterior end of the blastopore which here 

 also is slit-like. It should be mentioned further of those two anal 

 cells that, in various Gastropods, they mark at an early stage and in 

 a striking manner the position of the anus (pp. 154 and 160). They 

 coincide in position with the two cells which, in Patella, appear be- 

 hind the blastopore (p. 125). The anterior end of the blastopore in 

 the forms just named, also, becomes the mouth, so that the relations 

 of the blastopore to the mouth and to the anus in these forms are 

 specially distinct and the apparently divergent condition of Paludina 

 is thus explained. 



Greater attention has been paid to the form and the transformations of the 

 blastopore in the Gastropoda than in other animals, and the subject there- 

 fore has received special consideration from us. It was not our intention to 

 give an exhaustive account of the observations made in connection with it 

 chiefly because these are to some extent unreliable. We have therefore 

 selected only such statements as seem to some degree well-founded, though 

 even these need more careful examination. It seems, however, to be proved 

 by these observations that, in the Gastropoda, there are relations between the 

 blastopore on the one hand and the mouth and anus on the other. We conse- 

 quently find, in the Mollusca, conditions similar to those previously met with 

 by us in the Arthropoda, in which class also, the mouth and anus are either 

 directly or indirectly related to the blastopore (cf. Vol. iii., p. 412). The condi- 

 tion of Paludina may recall the Echinoderms, in which the blastopore passes 

 direct into the anus (Vol. i., p. 359). The condition of the Gastropods is, how- 

 ever, in any case, to be| traced back to corresponding processes met with in 

 the formation of the Annelidan TrocJiophore (Vol. i., p. 265). In the latter, the 

 blastopore at first lies at the vegetative pole of the embryo. It then extends 

 and occupies the whole length of the ventral surface which, however, is not 

 very great. When it closes from behind forward, its anterior end passes over 

 into the mouth, this latter lying behind the pre-oral ciliated ring, as in the 

 Gastropoda. The anus, however, arises at the posterior end of the larva which 

 previously corresponded to the vegetative pole and thus to. the position of 

 the blastopore. The conditions here are thus evidently very like those in the 

 Molluscs. 



