101 



Arachnoides placenta and regarded as a kind of gizzard; the stomach and 

 rectum are somewhat peculiarly shaped, especially the rectum with its 

 conspicuous widening. Of course, it is impossible to decide how much of 

 this is due to contraction on preservation, but it is evident that the shape 

 is not that of a simple oval sac. The ventral and dorsal transverse rods 

 give the body a characteristic prominence on the ventral and the dorsal 

 sides. - It was observed that this larva has the habit of swimming close 

 to the surface of the water; often 

 it comes up and touches the surface 

 film with the arm points, where- 

 upon it suddenly sinks as if it had 

 got a shock. 



The skeleton (Fig. 40) is of the 

 usual Clypeastroid type. The body 

 skeleton forms a basket structure, 

 the body rod and recurrent rod form- 

 ing together at their posterior end 

 a large, fenestrated plate, which is, 

 however, unusual through the holes 

 in it being very small, so that the 

 plate is nearly compact. There is both 

 an inner (upper) and an outer (lower) 

 ventral transverse rod. The postoral 

 and posterodorsal rods are fenes- 

 trated, the latter with distinctly 

 smaller holes than the former; both 



are thorny, the posterodorsal, how- Fig. 40. Skeleton of the larva of Echinarach- 



ever, almost exclusively along the """ oxentricus. side view. %. Letters as 



in fig. 20. 



outer side. The anterolateral rods 



are also rather much thorny. The basal part of the anterolateral rod has 

 a small process with which one of the basal processes of the postero- 

 dorsal rod joins so that here is formed a sort of articulation about which 

 this rod may move. Whether this is a constant feature, I do not, however, 

 venture to ascertain. - In some cases I have found some irregularities 

 in the body skeleton, as in that figured, where the lower ventral transverse 

 rod has some branches, one of them proceeding, like an extra recurrent 

 rod, downwards to the basal plate, with which it joins. 



This larva resembles to a striking degree that of Echinaraclwius par ma. 

 It is not easy to designate the characters by which to distinguish them, 

 especially because Fewkes does not give any detailed and exact figures 

 of the skeleton of the latter larva. It appears, however, that the plate 



