lO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. ^2 



About the ventral surface there are five long arms typically forking 

 at the base, thus representing biramous ventrolateral appendages and 

 calling to mind the biramous thoracic appendages of the barnacles. 



There can be no question of any direct homology between the cirri 

 of crinoids and the antennae of barnacles, or between the thoracic 

 appendages of barnacles and the arms of crinoids, but both sets of 

 organs have the same functions and the same location in each group, 

 and are constructed on a similar plan, so that it is not impossible to 

 regard them as parallel manifestations of the same ancestral ap- 

 pendicular plan, a plan not occurring in the animal kingdom outside 

 of the arthropods and crinoids. 



CHARACTERS OF A HYPOTHETICAL CRINOID WITH MID- 

 SOMATIC DEVELOPMENT ONLY 



Let us imagine a crinoid with entirely mid-somatic development 

 and with entire instead of half somites, that is, with bilateral instead 

 of radial symmetry. We would have a body composed of five broad 

 somites each covered with a broad arched plate (tergum) to the edge 

 of which is articulated a flap (oral, corresponding to a pleuron) ; 

 within this would be the radial (epimeron) at the inner edge of 

 which arises a biramous appendage. A body with five terga and five 

 pairs of biramous appendages with their bases covered by pleura 

 from which they are separated by epimera would certainly be con- 

 sidered as crustacean in character, and if it were attached by the head 

 end, with the mouth upward, it would unhesitatingly be pronounced 

 a barnacle, its deficiencies and anomalies of organization being as- 

 cribed to degeneration. 



THE CRINOIDS AND THE BARNACLES 



The crinoid develops from a highly anomalous larva, with a so- 

 called vestibule suggesting a partial development of a bivalved cover- 

 ing, which attaches itself by the anterior end like the cypris larva of 

 a barnacle and turns a half somersault bringing its mouth upward 

 and opposite the point of attachment, also like a barnacle ; so far its 

 development equally well suggests that of a polyzoan from a cypho- 

 nautes larva ; but in its further growth it develops a superficial 

 skeleton as does an arthropod of a sort already seen in rudimentary 

 form in the barnacles, with the chief nerve cords, which are highly 

 developed and arthropod-like, running over or just within its internal 

 surface as in the arthropods, and uniramous anterior (oriented from 

 the central nerve mass) and biramous ventrolateral appendages, 



