68 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



occurs during the metamorphosis of Echinoid larvse, the entoderm and the 

 other larval tissues undergo a process of histolysis during the metamorphosis, 

 and the small cells filling the lumen of the entoderm are the phagocytes 

 which produce the histolysis. The fact that they occur also in the mesoderm 

 and upon the whole in all the tissues of the larva is in good accordance with 

 this interpretation, while it is unintelligible from the explanation given by 

 Seeliger. Also, on purely logical grounds, such a multiplication of cells in 

 order to serve as food is quite unreasonable. 



It is noticeable that the phagocytes were found to occur in much smaller 

 numbers in Tropiometra than in Antedon and Isometra, but it is hardly 

 possible to give an explanation of this difference at the present stage of our 

 knowledge of Crinoid development. 



7. THE PRIMARY GONAD. 



While there is no reason to enter on a discussion of the development of 

 the coelom and its derivatives, the chambered organ and the axial gland, the 

 present researches being entirely in accordance with the observations of 

 Seeliger in this regard, there is something to say concerning the formation 

 which has been designated as the "primary gonad." 



It is the merit of Russo to have called attention to this organ. In his 

 "Studii su gli Echinodermi" (pages 10 to 14) he shows that in the anal 

 interradius there appears a small, distinctly limited group of cells, developing 

 in the mesentery. It corresponds exactly to the developing gonad of the 

 Holothurians, and Russo therefore concludes that it is really a gonad. But 

 while in the Holothurians it develops into the definitive genital organ, it 

 disappears completely in the Crinoid before the detaching of the young 

 Crinoid from the stalk, and a new, secondary gonad is formed, viz, the axial 

 organ, from which the definitive genital organs in the pinnules develop. 53 

 The primary gonad is an organ of the greatest morphological value, from the 

 presence of which it may be concluded that in the ancestors of the Crinoids 

 there was a single genital organ in the anal interradius. Evidently this 

 was the case in at least some of the Cystids, as may be concluded from the 

 presence of a pore besides the madreporite between the mouth and anal 

 opening, especially in the most primitive of all Cystids, Aristocystis. We 

 have herein an additional proof that the Crinoids are derived from the 

 Cystids. The existence of this primary gonad in the Crinoids also indicates 

 a closer relationship between the Holothurians and the Crinoids, while the 

 Asteroids, Ophiuroids, and Echinoids stand more apart. 



My own observations regarding this primary gonad are in accord with 

 those of Russo. The less satisfactory histological preservation of my material 



63 Russo (p. 12) further states that a new group of sexual elements develop from the peritoneal cells at 

 the esophagus, which forms a series of hollow genital tubes ("cordoni genitali") round the esophagus, that 

 connect with the axial organ. From this compound structure the genital tubes of the arms and pinnules 

 proceed. I have not been able to verify these observations on the material available for the present studies. 



