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BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



and in outline is circular or stellate. Locomotion is in any direction, except in 

 certain highly specialized types. The number of metameres is always five. Asym- 

 metry has affected the whole body so that one-half has become entirely atrophied 

 and the remaining halves of the five metameres haye curved about and, the an- 

 terior and posterior ends joining, have formed a radially symmetrical body. 



Eggs and segmentation. 



In most crustaceans the egg is enclosed in a tough chitinous membrane, and 

 the development is of the so-called centrolecithal or peripheral type ; but within the 

 group complete and equal division of the ovum similar to that of the annelids also 



occurs, and all intermediate types are 

 found. In the echinoderms total seg- 

 mentation ordinarily occurs; but in the 

 crinoids the egg is enclosed in a tough 

 membrane resembling that in which the 

 egg of most crustaceans is enveloped, at 

 the same time being attached to the pin- 

 nules of the mother in the same way 

 that thg egg of many crustaceans is at- 

 tached to the abdominal appendages of 

 the mother, and in Antedon adriatica 

 (the only species except the closely 

 allied Antedon mediterranea in which the 

 early developmental stages are ade- 

 quately understood) where there is a 

 relatively large amount of yolk we find 

 more than a hint of the centrolecithal 

 development so characteristic of the 

 arthropods. 



Development of the Inn-::-. 



FIG. 79. DORSAL VIEW OF THE TYPE-SPECIMEN OF COMA- 

 TULA PURPUREA FROM AUSTRAUA, SHOWING THE CIRRI 

 CONFINED TO THE INTERRADIAL ANGLES OF THE CENTRO- 

 DORSAL. 



The quotation from Professor Patten 

 preceding clearly indicates the very 

 close correspondence between the de- 

 velopment of the larvae of the echinoderms and that of the larvas of certain types 

 of crustaceans. It is sufficient here to note the fact that the larvas of the echino- 

 derms in their development pass through a striking metamorphosis, accompanied 

 by a remarkable histolysis, and a more or less pronounced metamorphosis which is 

 exactly comparable to it except for the absence of any change in the symmetry, 

 and a similar histolysis, occur in most arthropods. 



Echinodermal skeleton. 



The singularly specialized skeletal system of the echinoderms, though very 

 diverse in its manifestations, presents when analyzed a certain uniformity of 

 character throughout the phylum; taken as whole, it is of a somewhat different 

 nature from that of any other group of animals. 



