LAMELLIBRANCHIA UNIONIDAE. 



These peculiar organs are believed to communicate to the larva the stimu- 

 lus produced by coming into contact with a fish, and thus bo give rise to the 

 muscular movements which cause the shell-valves to close and the larva to 

 become hooked on to the host. 



There are a few more important organs to be mentioned in 

 connection with the further development of the larva of the 

 Unionidae, the first of these being the powerful adductor muscle 

 of the shell. This arises very early through the increase in number 

 of the rnesoderm-cells, which are still only slightly differentiated 

 [larval mesenchyme, LILLIE] (Fig. 23 (7, sm), these cells lengthening 

 and becoming attached to the shell-valves. The short but broad 

 muscle thus passes through the body-cavity from one valve to the other 

 (Fig. 25 5, sm). Besides this large muscle, there are a number of 



B. 



J 



9 -' 



*" SO. 



sm 



FIG. 25. Older embryo (within the egg-envelope) and free larva (Ulochidium) of A/w- 

 donta (after SCHIEBHOLZ and FOREL). /, larval filaments ; g, lateral pits ; s, shell ; 

 sh, shell-hooks ; sm, adductor muscle ; so, tufts of setae representing the sensory 

 organs ; w, ciliated area. 



other weaker muscles in the form of long mesoderm-cells attached in 

 various directions to the ectoderm, like the muscles which, in the 

 Troehophore, bring about the contractions of the larval body. 

 SCHIERHOLZ [and LILLIE] ascribe to the continuous contraction of 

 these muscles the withdrawal of the central part of the embryo above 

 mentioned. There are also, according to SCHMIDT, special muscles 

 in the form of modified mantle-cells connected with the shell-hooks. 

 A peculiar and characteristic larval organ arises in the median line 

 between the two halves of the mantle as an imagination of the 

 ectoderm (RABL). It grows inwardly as a long glandular tube which 

 coils several times round the adductor of the shell and secretes a 

 filament of tough substance which projects from the aperture of the 

 gland (Figs. 25 B and 26 A,f). This organ has been regarded as 



