98 



however, not been distinctly observed. — Across the edges of the anal 

 area a fairly regular series of nuclei is seen, evidently representing the 

 nervous system, as observed in the larva of Echinocyamus^). A very 

 distinct hne a little below the postoral band indicates the lower limit of 

 the suboral cavity. The lower end of the posterodorsal rods are con- 

 nected by what would appear to be a strand of muscles, these arms being 

 thus evidently movable. 



Fig. 39. Skeleton of the larva of Arachnoides placenta, ^"/i. A. from the ventral side; 

 B. side view. vtr.*. lower ventral transverse rod. Other letters as in fig. 20. 



The skeleton (Fig. 39) is of the typical Clypeastroid structure, the body 

 skeleton forming a basket structure, which develops in the posterior end 

 of the body into an irregular, fenestrated plate. The body rod and recur- 

 rent rod otherwise nearly smooth, except towards the end, where the 

 fenestration begins. From the body rod proceeds, besides the usual ventral 

 transverse rod, another similar rod somewhat lower down, so that there 

 is a double set of ventral transverse rods. The upper one, corresponding to 

 that normally occurring in Echinoid-larvse^) passes somewhat inwards, 



1) Th. Mortensen. Notes on the development and the larval forms of some Scand- 

 inavian Echinoderms. Vid. Medd. Dansk Naturh. Foren. Kebenhavn. Bd. 71. 1920, p. 156. 



'■) Also in the larva of Echinocyamus pusillus we may find both these ventral transverse 

 rods, only the upper one appears to be quite inconstant. It seems beyond doubt that it is 

 the upper one which corresponds to that normally occurring in Echinoid-larvse, while the lower 



