190 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



(loveloj)ed lobes in this lobod ganglionic mass is five on either side of the thorax, 

 each of the five corresponding to one of the large ambulatory appendages. In 

 the crmoids an identical lobod ganglionic mass occurs which also consists of five 

 lobes and represents one-half uf the lobod ganglionic mass in the decaj^ods. 



In the primitive Crustacea there are two parallel nerve cords running along 

 the ventral surface of the body from the suboesophageal ganglion, which are con- 

 nected at intervals by transverse commissures. The five primary nerves in the 

 crinoids immediately upon leaving the central nerve mass divide into two which, 

 like the ventral nerves of the primitive Crustacea, are connected at intervals by 

 commissures. In the docephalized crinoitls each of the five primary nerves repre- 

 sents one of the five nerves leading to the ambulatory appendages in the decapods; 

 but the replacement of the anteroposterior elongation of the body as seen in the 

 Crustacea by an enormous (now radial) development of each of the five half 

 metameres of which the crinoid body is composed has been accompanied by the 

 projection upon each of the five nerve cords running to the five (half) metameric 

 divisions of more or less of the characteristics of the entire crustacean ventral 



nerve cord. 



Eye. 



The eye in asteroids is a modified tentacle bearing numerous little cups lined 

 by sensitive and pigmented cells containing clear fluid and covered by cuticle; 

 the tentacle itself is a degenerate or very liighly specialized appendage which 

 originally corresponded to the metameric appendages of the crustaceans. The 

 replacement of an excised stalked crustacean eye by an antenna suggests that the 

 stalk of the crustacean eye may be in reality originally a metameric appendage; 

 if this be so the correspondence between the crustacean and asteroid eye is most 



remarkable. 



Sensory setae. 



The sensory setae of the crustaceans are possibly represented by the sensory 

 setse on the tentacles of the crinoids. 



Excretory organs. 



Well-defined excretory organs homologous with the nephridia of the annelids 



do not occur in the echinoderms; the excretory organs in the crustaceans are 



localized and segregated, being represented as "green glands" behind the base of 



each of the antennae. 



Genital ducts. 



In the crayfish {Astacus) the genital ducts open to the exterior through the 

 protopodite of the thoracic legs, of the last pair in the male, and of the second 

 ambulatory pair in the female; in the ochinoids they open through pores in the 

 genital plates which represent the protojjoditos of the thoracic legs in the crustaceans. 



Ccelom. 



In the crustaceans the true or primitive ccelom is always small in the adults, 

 and the apparant body cavity is of secondary origin, possessing in a great part a 

 blood carrying or vascular function. In the echinoderms the true or primitive 

 ccelom forms (1) the water vascular canals and (2) the true ccelom. 



