762 SUMMARY OF CURRENT RESEARCHES RELATING TO 



US to doubt that they then fall into the space which extends between the 

 gland and its envelope, and which, as has been shown, opens into the peri- 

 haemal circle ; from the perihremal circle the elements, thus formed, can 

 easily pass into the coelom. 



If this is really what happens in the madreporic gland, the same 

 phenomena ought to obtain, though in a much less important manner, in 

 the other parts of the vascular system. Throughout the vascular trunk 

 cells must be developed, which pass directly into the perihsemal canals, 

 for the same cells and the same arrangement are found in these as in the 

 madreporic gland. 



Although it does not seem to be correct to speak of such a system as 

 this as vascular, yet it appears to be well to retain the name provisionally, 

 so as to design among Echinoderms generally that collection of structures 

 which appear to be homologous throughout the group ; but it must be 

 remembered that great differences may be caused by the varying importance 

 of different portions of the system, and by the great development in some 

 of parts that are not present in other members of the sub-kingdom. 



Leaving aside the Holothurians, the author proceeds to institute a com- 

 parison between the circulatory system of Ophiurids and that of other 

 Echinoderms. The Asterida differ so far that the perihaBmal canals, in 

 which the different parts of the circulatory system are differentiated, com- 

 municate with other lacunse, developed in the dorsal surface of the test, 

 where they form an aboral ring. Owing to the situations of the madreporic 

 plate and of the genital organs, no such system of lacunae could be 

 developed in Ophiurids. 



In Echinoids the important difference is that the vascular trunks are 

 placed outside the psrineural spaces and do not always accompany the 

 nervous system ; the differences in the form of the vessels, and the mode of 

 communication between the aquiferous and vascular systems are of slight 

 importance. The arrangement of the vascular system in Echinids is too 

 like that of Asterids and Ophiurids for us to doubt the essential homology 

 in all these classes. 



The circulatory system of Crinoids, as made known to us by the works 

 of Perrier and of Vogt and Yung, is very different, but may nevertheless 

 throw some light on that of the groups just considered. It has been shown 

 that in the larva of Comatula, the dorsal organ gives off a bud which 

 penetrates into the arms, and developing in the pinnules forms the sexual 

 products ; the dorsal organ is, then, a kind of central stolon, from which 

 the gonads arise. 



In Echinids and Asterids the madreporic gland becomes continuous 

 with a circular system of canals from which branches pass off to the genital 

 organs. The question whether we have not here indications of more close 

 relations than have been suspected must be answered after a study of the 

 development of the madreporic gland and the gonads of Asterids and 

 Echinids. 



Radial Symmetry of Echinoids.* — Dr. W. Haacke discusses the old 

 question of the radial or bilateral symmetry of Echinoids. (1) He shows 

 in the first place that it is no direct corollary of Loven's law that these 

 animals are really bilateral. What Loven's researches distinctly proved 

 was the complete homology of the parameres in all Echinoids with the 

 exception of the Perischo-echinidae, and that Sjpatangus has a distinct and 

 constant median plane. But this does not prove the radial symmetry of 

 Echinoids. 



* Biol. Centralbl., vii. (1887) pp. 289-94. 



