ECHINODERMATA 461 



enterocoeles — should be confirmed. This would indicate an approach 

 to segmented forms. But in this connection one is involuntarily re- 

 minded of the condition of the larva of Balanogloxsus, in which, according 

 to Bateson, an internal segmentation is expressed by the establishment 

 of three pairs of coelomic sacs (comp. Fig. 167, p. 380). Even in its 

 external shape the Tornaria of Balanoglossus seems to possess a certain 

 resemblance to the Echinoderm larvfe. Added to this is the fact that 

 the so-called water-vascular vesicle of Balanoglossus may exhibit a bipar- 

 tite fundament. Also the water-vascular vesicle of the Echinoderms in 

 certain cases (especially in the Ophiuroidea, and occasionally in the 

 Asteroidea) is said to be begun as a i^aired structure (Metschnikoff). 

 Should this statement (hitherto held to be insufficiently authenticated) 

 be confirmed, then one would be able to compare the fundament of this 

 system of organs — so important for the Echinoderms, but so obscure as 

 regards its phylogenetic origin — with an embryonal excretory apparatus 

 (primitive kidney). This view is supported by the discoveries of P. und 

 F. Sarasin (No. 47), who explain the glandular structure, which opens to 

 the exterior at the same time as the stone canal — the so-called heart of 

 the sea-urchin — as an excretory organ communicating freely with the 

 body cavity by means of an open ciliated funnel. The assumirtion of 

 the Saeasins that the excretory system is the more primitive, and the 

 water-vascular system, with its locomotor function, is only an organ 

 derived from it, seems to be one that is required by the natural course of 

 events. 



Literature. 



1. Agassiz, a. Embryology of the Starfish. Cambridge. 1864. Also 



as Mem. Mus. Comp. Zoöl. Harvard Coll. Vol. v. 1877. 



2. Agassiz, A. On the Embryology of Echinoderms. Mem. Amer. 



Acad. Arts and Sciences. Vol. ix. 1867. 



3. Agassiz, A. Eevision of the Echini. Illustr. Cat. No. 7 { — Mem. 



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