REPOllT ON THE HEXACTINELLIDA. 17" 



between these there are pit-like depressions of irregular form (PL CII. fig. 1). The 

 summit of each of these projecting knots generally bears the apertui'e (0'5 to 1 mm. in 

 breadth) of an efferent canal. Similar apertures occur in varying abundance between the 

 nodes. The firm basal portion of the specimen is quite destitute of soft parts for a 

 distance of 2 to 3 cm., and must have been quite dead for some time (PI. CII. fig. 1). 



These projecting ridges above referred to are readily brought into relation with the 

 macerated skeletal structure exhibited by the lower portion of the other specimen 

 (PI. CII. fig. 2). This fragment shows very distinctly that on the outer convex surface 

 there is a reticulate framework, enclosing long, oval or angular meshes, and composed of 

 beams which lie parallel to the smooth arched surface. Internally there lies a system of 

 approximately parallel, or rather somewhat fan-shaped divergent longitudinal ridges which 

 measure 1 to 2 mm. in breadth, and now and again divide. Between these there are 

 longitudinally disposed canals of equal breadth, which are covered internally by a layer 

 of beams for the most part transverse. From this latter layer numerous round 

 perforate swellings and internally open tubes project into the interior. 



The round excurrent apertures of the efferent canal system, which measure about 

 2 to 3 mm. in width, are disposed in longitudinal and transverse rows intersecting at 

 right angles (PI. CII. fig. 1). 



From the macerated skeleton, or better still, from longitudinal and transverse sections 

 of the whole body, it is clear that the latter consists of a much folded plate 2 to 2"5 mm. 

 in thickness, in which the bounding surfaces are formed by the sieve-like dermal and 

 gastral membrane, while between these the ranch folded chamber layer is supported by 

 means of the familiar trabecular framework (PL CII. fig. 3). Many of the entrance 

 apertures of the aflereut canals are closed by a second external membrane split 

 off from the dermal. In its peripheral portion this membrane is supported by the sub- 

 dermal trabecular framework, but is subsequently quite freely expanded (PL CII. fig. 3). 



The dictyonal framework, which penetrates throughout the entire sponge body, 

 consists of moderately large simple hexacts which are soldered together, or occasionally 

 connected by synapticula in an irregular fashion. 



Only in a few regions, and especially in the neighbourhood of the dermal surface, have 

 the dictyonal hexacts the typical form with straight rays disposed at right angles to 

 one another, on which account I had formerly referred this genus to the Dictyonina.' 

 But they are usually curved and elongated in very various ways (PL CII. fig. 3), 

 so that an intersection and fusion of the rays of adjacent hexacts may occur at 

 very various angles, and the dictyonal framework in fact resembles rather the skeletal 

 framework of several Lyssaciua than the ordinary type of Dictyonina. The whole 

 skeletal beam is uniformly thick and cylindrical, with numerous irregularly scattered, 

 minute pointed spines (PL CII. fig. 3). 



1 tiber den Bau und das System der Hexactinelliden, Ablmndl. d. 7;. Preuss. Akad. Berlin, 1886, p. SO. 



(ZOOL. CHALL. EXP. — PART LUI. — 1887.) Ggg 23 



