■38 I. IJIMA : HEXACTINELLIDA. I. 



so-called gastral cavity (paragaster, cloaca, atrium, &e.) in direct 

 communication witli the exterior ; the lower end having a tuft 

 of spicules which serve to anchor the body in the substratum. 

 The oscula at the upper end convert that part of the body-wall 

 into a structure appropriately called the sieve-plate. The inflow 

 of w^ater takes place from tiie entire external surface, which is in 

 fact minutely and thoroughly perforated — that of the sieve- plate 

 beams not excepted ; the outflow takes place through the entire 

 internal surface ; and the final exit is through the oscula, i.e., 

 the sieve-plate meshes. 



That the sieve-plate is only a modified section of the general 

 body-wall follows from its spiculation, which, as long since 

 known (Marshall '75, p. 200; Schulze '95, p. 42), is fundamen- 

 tally the same as in the lateral wall, as well as from the presence 

 of flagellated chambers in the beams in exactly the same dis- 

 position as elsewhere.'"' 



The above primitive form has been met with by myself as 

 an ontogenetic stage in the post-embryonal development of M 



*It will be i^een that I have u.Sfd tlie terra osiculum in the sense of any single optning 

 in tlie parietes which serves as an exit for water from the gastral cavity, instead ofcalliu.;? 

 by tliat name, as has been done by several previous writers, the entire superior body-end 

 occupied by t!ie sieve-plate. In view of tlie above noticed nature of the sieve-plate and of 

 tlie presence of morphologically and physiologically identical openings on the lateral wall, 

 the usage I have adopted of the term seems tlie most conformable to the real circumstances 

 and, at all events, the best adapted for tlie purpose of description ; although it can not be 

 denied that the entire sieve-plate area of Eupleclella, &c., may exactly correspond to the 

 single, large, terminal osculum of certain other genera. 



So fiir as concerns the mode uf origin of the sieve-jjlale, my view completely concurs 

 with that expressed by MiNCiilN (Quart. Jour. Microsc. Sei., N. S., Vol. XXXIII, p. 258). 

 There can be no doubt whatever that the Euplectellid sieve-plate is formed by a breaking 

 through in several places of the gastral cavity to the exterior. As regards its function, the 

 writer just mentioned lias remarked that it may be of use in guarding against the intrusion 

 of animals into the gastral cavity, which opinion is very likely true. AVhereas, Kellek's 

 (Zeitschr. f. wiss. Zool., LH, p. 362) idea that it is an arrangement for keeping off' mud- 

 particles, which are assumed to be continually descending from the upper layer of waters, 

 seems to luck all grounds of plausibility so far as tlie matter specified is concerned. 



