132 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



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

 certain higlily specialized types. Tlio number of metameres is always five. Asym- 

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

 and the remaining halves of the five metameres have 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 cliitinous membrane, and 

 the development is of the so-called centrolecithal or peripheral type; but witliin 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 the 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 larvie. 



The quotation from Professor Patten 



Fio. 79.— Dorsal view of the type-specbien of Coma- 

 tula PURPUREA FROM AUSTRAUA, SHOWING THE CIRRI 



CONFINED TO THE iNTERRADUL ANGLES OF THE cENTRo- preccdiug cleai'ly iiidicatcs tlic vci'y 

 ''°^^'" close correspondence between the tle- 



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

 of cnistaccans. It is sufficient here to note the fact that the Inrvse of the echino- 

 derms in their development pass through a striking metamorphosis, accompanietl 

 by a remarkable histolysis, and a more or less pronovmced metamorphosis wWch is 

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

 and a similar histolysis, occur in most arthropods. 



Echinodcrmal skeleton. 



The singularly specialized skeletal system of the ecliinoderms, though very 

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

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

 nature from that of any other group of animals. 



