150 Mr. E. Billings on the Structure of 



Miiller, Agassiz, Thomson, Carpenter, and others, I should 

 say that in this class the most striking resemblance is that 

 which occiu-s between the adult stages of the Cjstidea, Blas- 

 toidea, and Crinoidea, on the one hand, and the embryonic 

 starfishes on the other. The structural character that has the 

 most important bearing on the subjects discussed in these 

 notes is, that in all four of these groups the mouth is situated 

 in one of the interradial areas, not in the ambulacral centre, 

 as it is in the adult forms of the existing Echinodermata. 



In Bipinnaria asterigera^ Sars, according to Miiller, the 

 digestive cavity is a subglobular sac, without any extensions 

 into the rays as there are in the adult starfishes. The oeso- 

 phagus (fig. 8, o) is a fleshy, consistent tube, with a large mouth 

 or pseudostome, j-^. It passes through the wall of the stomach 

 by an opening somewhat smaller than the mouth, and situated 

 in one of the inteiTadial spaces at m. The madreporic plate {f) 

 and the sand-canal (e), the latter holding the convoluted plate 

 (when it occurs), are situated above the orifice (>>i), and between 

 it and the ambulacral centi'e (c). The circular space at c is un- 

 doubtedly the homologue of the central space in the apex of 

 Nucleocrinus (figs. 3 & 5) and of Codonites (figs. 10 & 11). It 

 is also the position of the mouth in the adult starfish ; but in 

 the larval stage it is completely closed by the soft external 

 skin and sarcode of the body. In the fossils it is also closed, 

 but by an integument of thin calcareous plates. The Bijnn- 

 naria is nourished by minute particles of matter diffused 

 through the water and drawn into the digestive sac through 

 the mouth and oesophagus by the action of interradial cilia. 

 I believe that all the fossil Crinoidea, Blastoidea, and Cystidea 

 ingested their food in this way, and without any aid whatever 

 from the arms or pinnulce. 



Perhaps there is no embryologist who will not admit that it 

 is possible for an animal like Bipinnaria to develope organs of 

 reproduction and propagate its sj^ecies, none of its other parts 

 making any further advance. Such an animal, with some 

 slight modifications, would not be very widely different from a 

 palaeozoic Crinoid. If the sarcodic body-wall were to be con- 

 solidated into a thin calcareous integument, with the mouth 

 even with the sm'face, the swimming-appendages aborted, and 

 the vent closed up, it would resemble the cup of an Actinocri- 

 nus (fig. 9 a). The lateral orifice would then be both mouth 

 and vent, as it is, at first (according to Prof. A. Agassiz, ' Sea- 

 side Studies,' p. 125), in the embryo of Aster acantliion hery- 

 linus. The ambulacral canals of Bipinnaria are the homo- 

 logues, in a general way, of those which are fomid beneath the 

 vault o{ Actinocrtnus^ and extend outward into the grooves of 



