1870.] BILLINGS— ON CRINOIDEA AND BLASTOIDEA. 189 



resemble, both ia external form and internal structure, the 

 embryonic stages of those of the same class at present existing. 

 Prof. Agassiz has long taught in his lectures and various publica- 

 tions, that this is especially observable in the Echinodermata. 

 Judging from the figures and descriptions of Muller, Agassiz, 

 Thomson, Carpenter and others, T should say, that in this class, 

 the most striking; resemblance is that which occurs between the 

 adult stages of the Cystidea, Blastoidea, and paleozoic Crinoidea,, 

 on the one hand, and the embryonic Star-fishes 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 Muller, the digest- 

 ive cavity is a sub-globular sack without any extensions into the 

 rays, as there are in the adult Star-fishes. The oesophagus, fig. 

 8, 0, is a fleshy, consistent tube, with a large mouth or pseudos- 

 tome, p. It passes through the wall of the stomach by an open- 

 ing somewhat smaller than the mouth, and situated in one of the 

 interradial spaces at ni. The madreporic plate, /, and sand canal, 

 e, the latter holding the convoluted plate (when it occurs), are 

 situated above the orifice, m, and between it and the ambulacral 

 centre, c. The circular space at c, is undoubtedly the homologue 

 of the central space in the apex of Mideocrinus, figs. 3 and 5, 

 tjnd of Codonites, figs. 10 and 11. It is also the position of the 

 mouth in the adult Star-fish ; but in the larval stage it is com- 

 pletely closed by the soft external skin and sarcode of the body. 

 In the fossils it is also closed, by an integument of thin calca- 

 reous plates. The Bipinnaria is nourished by minute particles 

 of matter diffused through the water, and drawn into the digestive 

 sack through the mouth and oesophagus by the action of internal 

 cilia. I believe [that all the fossil Crinoidea, Blastoidcca and 

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

 whatever from the arms or pinnulte. 



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 species, none of its other parts 

 making any further advance. Such an animal, with some slight 

 modifications, would not be very widely difi"erent from a paleozoic 

 Crinoid. If the sarcodic body wall were to be consolidated into a 



