1570 THE VOYAGE OF H.M.S. CHALLENGER. 



arachnoiclal veil or mantle, composed of thousands of very fine, hollow, tangential needles. 

 The skeleton therefore is incomplete, without any direct connection between the isolated 

 pieces, just as in the preceding Cannorrhaphida, liut the latter never possess the large, 

 hollow, cylindrical, radial tubes, which are characteristic of all Aulacanthida. 



The spherical body of the Aulacanthida has usually a diameter of 1 to 2 mm., and 

 including the radial tubes, of 4 to 5 mm. or more. Some species are very common and 

 cosmopolitan, and some genera contain numerous species, distributed widely over all oceans. 

 In spite of their considerable size and wide distribution, only one species of this great family 

 has been hitherto known, having been discovered by me at Messina in 1859, and described 

 in my Monograph as Aulacantha scohjmantha (1862, p. 263, Taf. ii. figs. 1, 2, and 

 Taf. iv. figs. 1-5). I there founded for it the peculiar subfamily Aulacanthida, and 

 annexed it to the ThalassicoUida. The same cosmopolitan species has been subsequently 

 observed at Messina by R. Hertwig, who first recognised the three openings in its 

 central capsule, and therefore united it with his Tripylea (Organism, d. Radiol., 1879, 

 p. 88, Taf ix. figs. 3, 4; Taf x. figs. 7, 10). The rich collection of the Challenger has 

 added an astonishing number of new and interesting forms of Aulacanthida, so that I 

 can describe here not less than six genera and fifty-eight species. The majority are 

 inhabitants of the colder parts of the South Pacific and South Atlantic, at great depths, 

 whilst a few species only are found in the tropics. 



The structure of the body in all Aulacanthida seems to be similar in all important 

 points; and the differences by which we are enabled to separate this great number of species 

 are mainly produced by diff'erences in the development of the radial tubes, their form and 

 their polymorphous apophyses. The entire body represents a rather firm jelly-sphere of 

 1 to 2 mm. diameter (rarely less or more) ; the peripheral layer of the spherical caljTuma 

 is rather clear and transparent, whilst its central part is dark and opaque, containing the 

 big phasodium and the enclosed central capsule. The diameter of the latter is usually 

 between O'l and 0'3, often 0-4 to 0-5, or even more. The gelatinous calymma, in the centre 

 of which the capsule is placed, always contains numerous, large, spherical or roundish 

 alveoles, similar to those of ThalassicoUa, and between them a delicate network of 

 sarcode (PL 102, fig. 1 ; PI. 103, fig. 1 ; PI. 104, fig. 1). 



The spherical surface of the calymma is nearly always protected by that characteristic 

 arachnoidal veil or mantle, which is composed of thousands of very fine tangential 

 needles, densely interwoven in aU tangential directions, but never directly connected. 

 They are wanting in a single genus only, in Aulactinium (PI. 101, figs. 6-8). This 

 genus, therefore, may represent a separate subfamily, the Aulactinida, whilst all other 

 genera protected by that mantle constitute the subfamily Aulographida. The tangential 

 needles always seem to have the same shape as I have accurately described, in 1862, of 

 Aulacantha scohjmantha. They are constantly smooth, very thin and fragile, but also very 

 elastic cylinders of silica, of equal breadth throughout their whole length, and seem 



