VIII ECHINODERMATA-^THE AXIAL ORGAN 445 



development. Further, the communication which always primitively 

 exists between the axial sinus and the aboral circular sinus of the 

 genital system is interrupted in adult Echinoids, the only known 

 exception to this rule being JEchinoci/nmiis jmnillus. 



4. Cpinoidea. — In the Comatulkla', an axial section of the body 

 cavity round which the intestine becomes coiled is said to exist. In 

 other Crinoids, such an axial sinus seems to be wanting, or else is filled 

 with connective tissue. There is, in adults, no connection between this 

 sinus and the stone canals. In the direction of the principal axis, 

 however, the axial sinus is traversed by a dorsal (glandular) organ, 

 which, although of somewhat different structure, no doubt corresponds 

 with the axial organ of other Echinoderms. This homology seems to 

 be established by the fact that the dorsal organ of the Crinoids shows 

 relations to the genital system similar to those of the axial organs in 

 the Echinoidea, the Asteroidea, and the Ophiuroidea to the same 

 system. 



■5. Holothurioidea. — In this class there is no axial sinus cut off 

 From the general body cavity. 



F. The Axial Organ. 



(Dorsal Organ, Heart, Pseudo-heart, Kidney, Plastidogenio Organ, Ovoid Gland, 



Lymph Gland.) 



No other organ of the Echinoderm has given rise to so many contra- 

 dictory statements as the axial organ. The names given in the above 

 heading, which have been gathered from various authors, show what 

 different functions have been ascribed to it. 



According to the most recent anatomical and ontogenetic researches, 

 the following points may be stated with some degree of certainty. 



(«) The axial organ lies on or in the axial sinus. 



(6) It is developed from the endothelium of the body cavity, and, 

 during early ontogenetic stages, it grows out in the shape of pro- 

 cesses, strands, or tubes, which, at certain definite points of the body, 

 become the gonads (ovaries and testes). 



(c) Further, in the adult animal, the axial organ is in most cases 

 still connected with the genital system, functioning, however (at least 

 in the Asteroidea, Ophiuroidea, and Ediinoidea), in all probability, as a 

 lymph gland. 



The Holothurioidea appear to possess no axial organ. 



In the Astrroideu, Ophiuroidea, and Echinoidea, the axial organ 

 consists of a network of connective tissue, in whose meshes (embedded 

 in blood f)lasm) round cells lie, which, by continual division, yield 

 lymph cor2:)uscles. 



1. Asteroidea. — The axial organ lies in the axial sinus, to the wall of which it 

 is attached by a mesentery. Below the madreporite it sends off a process into a 

 small cavity, which is completely cut off from the axial sinus. Further, it pro- 



