NOTES ON BCHINODEEM MORPHOLOGY. 603 



These cells were supposed by Perrier, and also by Aposto- 

 lides, in the case of the Ophiurids, to be grouped into columnar 

 acini, as in the case of compact glands like the liver. Koehler 

 was at first inclined to take the same view of their arrangement, 

 but subsequent investigations led him to give the description 

 of the structure of this organ, which has been summarised 

 above. Several so-called lacunse are visible in sections through 

 the gland. Some of them correspond to the numerous anas- 

 tomosing canaliculi which unite into its excretory duct. But 

 others, especially those in the peripheral and lowest parts of 

 the gland, are filled with coagulum. These represent the 

 lumina of the vessels which ramify on its surface, and are con- 

 nected with the " glandular canal" rising from the oral blood- 

 vascular ring. The structure of the upper part of this canal 

 is essentially similar to that of the gland itself; but towards 

 the peristome the trabecular structure becomes less marked, 

 and lacunar spaces lined by an epithelium relatively more 

 prominent. Various transitional stages may be found between 

 the cellular elements of the ovoid gland and the epithelial cells 

 lining the lower portion of the glandular canal, and Koehler 

 regards the former as a modification of the latter. 



The glandular canal and parts of the ovoid gland of the 

 Urchins thus consist of numerous small vessels with an epithe- 

 lial lining, while the cells of other parts are more irregularly 

 arranged among fibres of connective tissue. This is not so 

 very different from Lud wig's account of the same organ in 

 Asterids and Ophiurids, the correctness of which has been 

 questioned by Perrier and Apostolides ; and it would seem, 

 therefore, that the designation '' central plexus" is not so very 

 inapplicable after all. " Plexiform gland" would, perhaps, be 

 better; but any appellations based upon its form, such as 

 "ovoid gland" (Perrier) or "piriform gland" (Apostolides), 

 are inconvenient when applied to other types of Echinoderms, 

 such as the Crinoids, in which the so-called ovoid gland breaks 

 up into a number of loosely-connected lobules. The organ in 

 question is most certainly not a heart, and may very probably 

 have something to do with the production of the brown 



