40 



LAMELLIBRANCHIA . 



vel 



mes.' 



The other fresh-water Lamellibranchs show the Trochophore form merely 

 as one of the stages of their embryonic development, and in them, as 

 compared with the marine Lamellibranchia, it has greatly degenerated. 

 The fact that this degeneration has taken place is made clear by the compli- 

 cated form of the alimentary canal. In Cyclas and Pisidium, as well as in 

 the Unionidae, the blastopore closes ; the ectoderm then becomes separated 

 from the archenteron, so that there is now an entirely closed entoderm- 

 sac. This latter only becomes connected again with the ectoderm at a later 



stage. This takes place 

 first through the stomo- 

 daeal invagination, whose 

 relation to the blastopore, 

 in consequence of the entire 

 obliteration of the latter, 

 has not been ascertained, 

 and then through the forma- 

 tion of the proctodaeunv* 

 When the entoderm-sac 

 thus becomes connected 

 with the ectoderm at two 

 points, the rudiment of the 

 alimentary canal is formed. 

 - - f This latter is probably 



composed of the same con- 

 stituent parts as that of 

 the TrochopJiore, although 

 its formation has been less 

 direct. 



The velum, that specially 

 important organ of the 

 Lamellibranch larva, is 



very much reduced in those forms which do not swim about freely at the 

 Trochophore stage. In Cyclas, all that remains of the ciliated apparatus of 

 the Trochophore is a small ciliated area extending above and below the mouth 

 and at its sides. ZIEGLER has homologised this ciliated area with the ad- 

 oral ciliated zone of the Trochophore, and believes that the part of the velum 

 connected with feeding is partly retained while the part chiefly connected with 

 locomotion and which was no longer used has completely disappeared. The re- 

 duction of the velum has led to a corresponding reduction of the larval muscles. 



It has already been mentioned that the larval kidney is found in 

 Cyclas. Of the other Trochophoral organs, ZIEGLER was only able 



* The statements that the biastopore becomes directly transformed into the 

 anus (as in Pisidium, BAY LANKESTER) require confirmation, since the more 

 primitive Lamellibranchia show an entirely different relationship (p. 31). 

 ZIEGLER'S account of the processes in Cyclas, indeed, seems to show that the 

 anus arises at the posterior end of the slit-like blastopore which has already 

 closed. A proctodaeal invagination seems in any case to be absent in this 

 form. 



FIG. 19. Embryo of Cyclas cornea at the Trocho- 

 phore stage (combined from E. ZIEGLER'S figures) ; 

 by, byssus ; eg, cerebral ganglion ; d, enteron ; 

 ,/, foot ; m, mouth ; mes, mesoderm ; mr, rudiment 

 of the mantle ; pg, pedal ganglion ; sd, shell-gland ; 

 vd, stomodaeum ; vel, velar area. 



