592 INVERTEBRATE MORPHOLOGY. 



stalked form for an ancestor, and to consider the Crinoids as approaching 

 it more nearly than any other recent forms. It has been suggested, with 

 no little reason, that the Cystoidea were the ancestors of the Crinoids and 

 perhaps of most of the other groups as well. A full consideration of this 

 point, as well as of the details of the conversion of the bilateral remote 

 ancestor into the radial form, would carry us beyond the scope of this work, 

 and reference must be had to special works treating of these questions 

 (P. and F. Sarasin, O. Butschli). 



As stated, the relationships of the Echinoderms to other groups is a 

 question which has not yet been satisfactorily settled. Attention may be 

 called, however, to the remarkable similarity of the jforacm'a-larva of 

 Balanoglossus (p. 606) to the Echinoderm larva, a similarity so great as to 

 suggest affinity. This suggestion may, however, be postponed until the 

 Tornaria has been described. 



SUBKINGDOM METAZOA. 

 TYPE ECRINODERMA. 



I. Class CRINOIDEA. Usually stalked ; with ten (or five ) arms, provided 

 with lateral pinnules, arising from the margin of the cup-shaped 

 body. Dermal skeleton well developed. 



In adult life free-swimming. Antedon, Actinometra. 



Fixed throughout life, stalk with numerous whorls of cirri. 



Pentacrinus. 



Fixed throughout life, slender stalk with cirri either wanting 

 or only on distal joints. Rhizocrinus, Calamocrinus, Hyo- 

 crin us. Thaumatocrinus. 



Fixed throughout life, stalk short and stout. Hulopus. 

 II. Class ASTEROIDEA. Free forms ; stellate or pentagonal in shape ; arms 

 containing caecal processes of digestive tract ; ambulacra limited 

 to oral surfaces. 

 Dermal skeleton reticulate ; no paxilla3 ; anus present. Aste- 



rias, Brisinga, Asterina, Zoroaster. 



Dermal skeleton of separate plates ; paxillae present ; no anus. 

 Astropecten, Lnidia. 



III. Class OPHIUROIDEA. Free forms ; stellate in shape ; arms not contain- 



ing cascal processes of the digestive tract ; ambulacra limited to 

 oral surface ; ambtilacral ossicles contained within the arms. 



1. Order Ophiurida. Arms unbranched ; madreporiform tubercle 



on mouth-shield. Ophiura, Ophiolepis, Amphiura, Opltiactis, 

 oplriotlirix, Ophioderma, Ophiomyxa. 



2. Order Euryalida. Arms usually branched ; mouth-shields not 



well developed ; usually several madreporiform tubercles. As- 

 trophyton, Trichaster. 



IV. Class ECHINOIDEA. Free forms; without arms; test composed of 



