NO. II ECHINODERMS AS ABERRANT ARTHROPODS CLARK II 



both sets of which have functions similar to those of the correspond- 

 ing appendages in the barnacles. 



Thus in spite of the utter dissimilarity in the details the broader 

 features of the structure of the crinoids and the barnacles as well as 

 of their development are seen to be similar, or at least comparable, 

 or perhaps it should be said that there is less divergence between 

 crinoids and barnacles than there is between crinoids and any other 

 organisms. 



THE CRINOIDS AND THE ABERRANT BARNACLES 



The crinoids represent a derivative from a branch of the same 

 arthropod stock that gave rise to the barnacles, but they have gone 

 much further; half of each of the five segments of which the body 

 is composed fails to develop so that the body is composed of five 

 half segments joined in a circle with the central organ of the ventral 

 nervous system at one pole and the mouth, which has moved poster- 

 iorly and come to lie near the anus, at the other ; the development 

 of the body structures after early youth suddenly becomes entirely 

 inter-somatic instead of mid-somatic ; the peristomal region becomes 

 enormously enlarged and extended, resulting in the formation of a 

 sort of lophophore ; and the appendages have been suppressed, or at 

 least appear in a very modified form late in life. Certain hereditary 

 tendencies show themselves after the animal has, so to speak, re- 

 covered from the profound ontogenetic shock resulting from the 

 loss of half its body, in the appearance of articulated uniserial tactile 

 and grasping organs at the neural pole (the original anterior end) 

 and of articulated biramous appendages used for food gathering or 

 for locomotion along the ventrolateral border. 



Crinoids, like barnacles, are sessile, pedunculated, attached by 

 hook-like processes, or unattached. 



The strong probability that the arthropod stock which by profound 

 modifications gave rise to the barnacles also gave rise to the crinoids 

 is indicated not only by the asymmetry in the Verrucidae, in which 

 the operculum consists of the scutum and tergum of one side only, 

 those of the other side being fused to form one half of the wall which 

 is completed, on the side of the movable opercular plates, by the 

 greatly developed and displaced rostrum and carina, but also by the 

 anomalous parasitic forms which have developed among the former 

 in which the aberrant features are so fundamental that they have 

 been thrust forward into the ontogeny so far as to modify profoundly 

 the form and structure of the naupHus. These forms also show that 



