REPORT ON THE HEXACTINELLIDA. 67 



and by the incurrent canals (PI. IV. fig. 4). The saccuhir chambers which lie adjaceut 

 to ouc another have either grown together laterally, at the points of contact, so as to form 

 narrow longitudinal stripes, or are connected by means of short beams (PI. IV. figs. 4, 6). 



The interspaces and canals extending from the apertures of the outer trabecular 

 framework, and passing in between the chamber's, are terminated by a connecting 

 membrane which is stretched, as a direct continuation of the chamber walls, between the 

 margins of the chamber orifices. 



The inner trabecular framework, on the other hand, together with the internal 

 perforated skin, which is known as the gastral miemhrane, enters the excurrent canals 

 and lines them as far as the orifices of the chambers (PI. IV. fig. 4). 



Without entering here more minutely into the histological relations of the soft parts 

 thus generally referred to, I would merely note, that in some cases I found, both in the 

 inner and in the outer trabecular framework, numerous sperm-balls, and also ova of 

 different sizes (up to 0"3 mm. in diameter), filled with round yolk-granules. In all these 

 ova it was peculiarly remarkable that the germinal vesicle which contained a large 

 nucleolus was protruded outwards, and lay on the surface in a round hollow pit-like 

 depression of the surface. 



The Skeleton. — The filagree-like lattice framework, which is readily obtained by 

 macerating older specimens, as also the spicules which lie freely in the soft tissue, are so 

 well and so accurately described by such earlier observers as Owen, Glaus, Marshall, 

 and Carter, that I content myself w4th referring to their excellent works, and wiU here 

 only briefly note the more important points in connection mth the architecture of the 

 skeleton, characterise the different kinds of spicules according to their form and 

 arrangement, and discuss, at greater length, some points which have hitherto been but 

 little noted. 



The groundwork of the entire continuous tube-skeleton consists of two layers of 

 beams crossed at right angles. One of these layers, the inner, is composed of circular 

 strands of fibres, while those of the outer have a longitudinal direction. Both together 

 form a lattice-work which is more clearly recognisable on the inner side, and which 

 exhibits quadrate meshes, varying from 3 to 5 mm. in breadth. Outside these two 

 layers of beams, and partly also between them, are two other systems of intersecting 

 fibres, which run diagonally to the former, and surround the tube in opposite directions 

 in oblique spirals. They are particularly obvious on the outer side of these quadrate 

 meshes of the main lattice-work which neither have parietal pores, nor are covered by 

 external ridges. These externally protruding elevations or ridges are supported by two 

 layers of steeply-set beams, which lie just beneath the two lateral surfaces, and which, like 

 the opposite rafters of a very steep roof, have their outer ends united at an acute angle 

 corresponding to the sharp edge of the ridges. They are also firmly fused laterally to 

 one another, and to the strong lattice-work of the tube (PI. IV. fig 3). These rafter-like 



