2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 'J2 



THE DOMINANT CHARACTERISTICS OF THE ECHINODERMS 

 The echinoderms are very anomalous forms, and their relationships 

 to other animals are masked not only by a highly perfected radial 

 symmetry, but also by a unique development of all the organs of the 

 body. Their outstanding features are the presence of a vascular, a 

 respiratory, and a superficial skeletal system, the last composed of 

 articulated (calcareous) elements, the absence of gill clefts, and the 

 sharp division of the body externally into (five radial) segments. 

 In these features they agree only with the arthropods. 



THE LARV-'E OF THE ECHINODERMS 



The unique larvae of the echinoderms, which are of very varied 

 types, vermiform, bean-shaped with five ciliated rings and a long an- 

 terior tuft of cilia, auricularise, bipinnariae, brachiolariae, plutei, etc., 

 aiid dift'er more or less widely among themselves in the details of their 

 development, are always at first bilaterally symmetrical, which may 

 be accepted as an indication that the echinoderms are derived from 

 bilaterally symmetrical ancestors. 



The larvae do not grow directly into the adults, but the latter for 

 the most part arise from new growth within the larval body, the 

 structures peculiar to the larvre being absorbed ; in a few cases the 

 development is direct. 



There is little in the structure or in the development of the echino- 

 derm larvae which is comparable to the structure or to the develop- 

 ment of the larvae of any other animals, and it is evident that the 

 extraordinary features exhibited by the adult echinoderms have been 

 projected so far forward in the ontogeny as quite to destroy the 

 value of the larvae as phylogenetic indices. 



THE CHANGE FROM MID-SOMATIC TO INTER-SOMATIC DE- 

 VELOPMENT IN THE CRINOIDS 



Perhaps the most interesting feature connected with the mor- 

 phology of the crinoids, and one which it is necessary especially to 

 emphasize in order to understand the relationships between them and 

 the other echinoderms, is the abrupt change in the regions of bodily 

 growth and extension which takes place beginning with the formation 

 of the arms. It is this sudden change from interradial to radial, or 

 from mid-somatic to inter-somatic, development which occurs at 

 the commencement of arm formation that has always proved the 

 chief stumbling block in the way of a correct interpretation of these 

 animals. 



