REPORT ON THE HEXACTINELLIDA. 45 



between the two is generally sharply defined by the peculiar spicules — marginalia — of 

 the oscular orifice (PI. XXXVIII. fig. 1). 



As in the dermal skeleton, so here, under the more superficial, deeper spicules occur, 

 that is, spicules further removed from the inner bounding surface, and more embedded in 

 the parenchyma. These are strong hexacts or pentacts with their radial rays disposed 

 at right angles to the inner surface, and with the four cruciate transverse rays parallel to 

 the same (PL XXII. figs. 2, 5). In harmony with the term hypodermalia, these may be 

 designated hypogastralia. I must, however, note that hypogastralia are often wanting 

 where hypodermalia are present, and the same is true of other typical spicules like 

 floricomes, amphidiscs, clavulse, scopulse, &c. It may be laid down as a rule that 

 the spicules of the gastral skeleton resemble the dermalia of the same Sponge in 

 general characters, but not in their special development, dimensions, and the like. 

 Thus, for example, the radial axis in one or two rays in the gastralia is frequently 

 well developed, while it is absent in the dermalia, and so hexacts in the gastral 

 membrane are often contrasted with pentacts in the dermal membrane (PI. LVIII. 

 fig. 2). In other cases the free ray is .short and broad in the dermal pinuli, but long 

 and thin in the gastral (PL XXV. fig, 3). Where the dermal clavulse exhibit a knob-like 

 extremity, the corresponding gastralia have long anchor-teeth (PL LXXV. fig. 2). 

 Dermal scopulae with pointed teeth are contrasted with gastral scopulse with button- 

 like teeth (PL LXXVII. fig. 2), and so on. 



As the dermalia of the outer skin do not usually pass into the aS"erent subdermal 

 spaces and canals, so the gastralia do not, as a rule, pass from the inner skin bounding 

 the gastral cavity into the efierent canals (PL XXI. fig. 2 ; PL LVI. fig. 2). In not a 

 few Hexactinellida, however, there is a development of peculiar canalaria, which lie on 

 the inner surface of the efi'erent canals, and appear to be a continuation of the gastralia ; 

 whether it is that, in the absence of a special continuous gastral skin, the efl:erent canals 

 open directly with large orifices into the gastral space, and the gastral skeleton simply 

 enters into the eflferent passages (PL XXXV. fig. 2 ; PL XXXVI. fig. 1), or that a 

 special gastral skin encloses the gastral space with a well-developed gastral skeleton, 

 while the efi'erent passages are, in addition, provided with similar spicules — canalaria 

 (PL XLVI. fig. 1). The canalaria are usually present only in the main stems and large 

 branches of the efi'erent canal system, but are not continued into the ultimate blind ends 

 (PL XXXVIII. fig. 1). 



Parenchymalia. 



While in the dermal and gastral skeleton there is always a very distinct and tj-jjical 



agreement in the position and arrangement of the spicules, this is not the case everywhere 



with the parenchymal skeleton. I think, however, that here also certain simple relations 



may be recognised as original, from which the less regular have been secondarily developed. 



