M. K. A. Zittel on Fossil Hexactinellida. 411 



themselves, moreover, affect the external appearance of the 

 fossil sponges in so high a degree that A. Komer and Pom el 

 have already given them what appears to me to be too much 

 systematic importance. 



2. The canal-system for the circulation of the influent and 

 effluent water, with the openings belonging to it, is of more 

 importance physiologically than the covering layers. 



In the Plexactinellida the walls of the sponge-body are 

 usually only of small thickness, and they enclose a very wide 

 central space of tubular, cylindrical, or funnel-like form. In 

 the trochiform, infundibuliform, and cylindrical Hexactinellida 

 the wide central space is best conceived as a common stoma- 

 chal cavity ; and consequently such sponges are to be regarded 

 as monozoic bodies ; the superior terminal aperture would then 

 be described as the osculum. In branched, lamelliform, or 

 nodose bodies, and in such as are composed of niffiandriform 

 tubes, the question of individuality is generally hard to settle, 

 as the apertures which have hitherto been usually regarded as 

 oscula are often distributed quite at random and frequently 

 appear scarcely to stand in connexion with gastral cavities 

 ( Ouettardia^ Phurostoma^ Pleurope) . 



The passages by which the water penetrates into and flows 

 through the sponge-body, in opposition to these oscula and 

 pseudo-oscula, are extraordinarily constant in their character, 

 and thereby furnish excellent systematic data. According to 

 Hackel*, the canal and water-vascular system is the most 

 significant, and physiologically and morphologically the most 

 important system of organs in all sponges. It determines not 

 only the most essential differences in the form of the body in 

 the various groups, but also more or less the structure and 

 form of the skeleton. 



In general the canal-system in the Hexactinellida exliibits 

 very simple conditions, which in many respects remind us of 

 those of the Sycones. Usually it is comjwsed merely of a 

 great number of simple, very rarely ramified canals, which 

 penetrate in a radial direction either perpendicularly or ob- 

 liquely into the wall. Only exceptionally the canals perforate 

 the whole thickness of the wall {Ajyhrocalltstes) ; much more 

 frequently they commence either on the outer or inner surface 

 of the wall, and terminate ctecally immediately within the 

 opposite surface ( Ventriculites^ Coscinopora, Hporadopyle^ &c.). 

 In this way, therefore, the usually round ostia of the two sur- 

 faces stand in alternating series. 



The circulation of water is effected in the simplest form 



• Die Kalkschwamme, i. p. 210. 



