350 



INVERTEBRATA 



CHAP. 



a misinterpretation of the early stages of development. Cyclas is 

 further remarkable for the fact that the coelom makes its appearance 

 as two vesicles situated at the sides of the intestine. These vesicles 

 become constricted into dorsal and ventral halves, but they meet one 

 another both above and below the intestine, and hence, in another 



FIG. 280. The Veliger larva of Yoldia 

 limatula, about three days old. (After 

 Drew.) 



a.p, apical plate ; U, position of blastopore ; 

 V, velum. 



FIG. 281. Longitudinal sagittal section 

 of the Veliger larva of Yoldia limatula, 

 three days old. (After Drew.) 



a.p, apical plate; c.g, rudiment of cerebral 

 ganglia (representative of cerebral pit) ; s.g, 

 shell gland ; st, stomach ; stom, stomodaeum ; 

 V, velar cells. 



way, the same end results as that attained in Dreissensia. The space 

 between dorsal and ventral halves forms the auricle of the heart, that 

 between the vesicles and the intestine forms the ventricle (Fig. 282). 

 The Unionidae give rise to an extraordinary larva, known as 

 a Glochidium ; it is devoid of mouth, velum, and foot, but provided 

 with a bivalve shell, and the lower borders of the valves are each 



