REPORT ON THE ECHINOIDEA. 93 



five openings to the interamljiilacral spaces ; this pouch confines the ahmentaiy canal 

 within a proportionally small area of the test, no single fold of the alimentary canal ex- 

 tending beyond half-way from the actinostome to the edge of the test (PI. XIV. fig. 2). 



The alimentary canal coming out from the jaws into the left anterior interambulacral 

 area (seen from the inside) runs just inside of the auricles, along the edge of the principal 

 plates to the right aml^ulacral space, where it turns back again, runs outside the auricles 

 completely round the actinostome to the right interambulacral space where it makes 

 another turn, and then, forming great loops in extending outwards in the interambulacral 

 spaces and shorter loops turned towards the actinostome in the ambulacral spaces, it 

 passes to the anal opening in the right anterior interambulacral space (PL XIV. fig. 2). 



Plate XII. fig. 2, shows for Fhormosoma hoplacantha the position of the alimentary 

 canal as it starts from the jaws and runs just inside of the auricles ; it also shows the 

 mode of attachment of the mesenteries of the horse-shoe pouch to the abactiual system, 

 and the complete separation by this returning fold of the space in which the ovaries are 

 placed from the poriferous zone, as is seen in PL XIV. fig. 2 on the actinal side. 



In this species of Phormosoma, as in the genus so far as has been observed, the plates 

 overlap in the opposite direction in the ambulacral and interambulacral spaces. 



In the interior view of the test of Phormosoma temie (Pis. XIV., XVIII." figs. 

 1-13) the lapping of the whole edge of the plate, so characteristic of Phormosoma, is 

 well seen, the double line round each coronal plate showing the extent of the overlapping, 

 and the double line along the outer edge of the ambulacral zone shows the extent of the 

 lateral covering of the edge of the ambulacral area. In Asthenosoma. it is only the ex- 

 tremities of the ambulacral and interambulacral plates which lap along the median hue 

 and along the line of junction of the two areas, whUe hi Phormosoma this lapping extends 

 along the whole horizontal edge of the plate in both areas. In Asthenosoma, viewed from 

 the outside, the outer and inner lower edges of the coronal plates pass under the upper 

 edges of the preceding plate in the interaml^ulacral areas, while in the ambulacral area it 

 is the upper edges of the plates which pass under the lower edges of the succeeding 

 plates counting from the actinostome. 



In the actinal system seen from the interior (PL XVIII.'^ fig. 6) the ambidacral tubes 

 pass under the auricles to plates immediately adjoining the teeth, where they are con- 

 nected by a circular tube (PL XVIII.'^ fig. 4). The ampuUae are large, and the tubes 

 leading to them distinct. A series of mesenteries separating the ambulacral from the 

 interambulacral system, extend from the base of the auricles to the actinal edge of the 

 actinal system. 



The structure of the actinal and abactinal systems in this family shows most satisfac- 

 torily the embryonic characters of the family and the intimate connection the plates 

 composing these two systems have, the one with the base of the actinal extremity of the 

 ambulacral system and the other with the abactinal plates of the interambulacral system. 



