202 



HARDWPCKE'S SCIENCE-GOSSIP. 



[Sept. 1, 3S66. 



less, under most circumstances, it is very difficult to 

 see this aperture in living specimens ; and having in 

 my examination commenced with these, I found that 

 it was only by very careful illumination with lamp- 

 light that I could thoroughly satisfy myself in the 

 matter. The following Camera Lucida sketch (fig. 



100 diameters. 



19S) was taken from a recent specimen of an im- 

 mature Drassus, the specific name of which I was 

 unable to determine ; but since then I have found 

 that the cast skins of spiders form admirable speci- 

 mens, for they not only show the opening (a) far 

 more distinctly than when connected with the living 

 animal, but they also present the fangs in a most 

 favourable position for examination. 



In most instances, and under these circumstances, 

 the opening may be detected without any trouble; 

 but without a more extended examination than that 

 which I have as yet made, I am not at all prepared 

 to state that the same arrangement exists in every 

 spider. Richard Beck. 



Chalk Markings. — In many chalk quarries the 

 faces caused by natural cracks are covered with 

 black seaweed-like markings. So like are they that 

 many think they are sea-weed impressions. But as 

 they only occur in natural cracks, I conclude that 

 they are the result of the percolation of water from 

 the surface earth charged with carbonaceous matter. 



Am I right? 



By way of experiment, I cracked a lump of 

 chalk artificially, put ink along the line of the 

 crack, and produced such a good imitation that a 

 friend was unable to distinguish between it and 

 the reality. 



I shall be much obliged if you will tell me what 

 the markings are caused by, and of what sub- 

 stance they are composed. W. T. 



CLAWS OP OPHIOCOMA ROSULA. 



WHILST examining the remains of an arm of 

 0. rosula after maceration in caustic potash, 

 I observed a considerable number of claw-shaped 

 bodies, similar to those shown in fig. 199. These 

 claws, as far as I have been able to ascertain, have not 

 been previously noticed. Professor E. Forbes, in his 

 " Monograph of the British Star-fishes," does not 

 mention them, although he minutely describes this 

 species. Being desirous of ascertaining the position 

 of the claws, I examined by the aid of the micro- 

 scope, an arm with the spines and plates in situ, and 

 found them attached to the lower margin of the 

 lateral ray-plate, near the base of the last t spine 

 (fig. 200). 



Fig. 199. Claws from Ophiocoma rosula x 130. 



These appendages have a cellular burr on the 

 lower or attached portion, the remaining portion 

 being smooth and glassy ; the inner margin of the 

 claw has a kind of spur or tooth curving downwards. 

 I am unable to say decidedly for what purpose these 

 appendages are intended ; but my impression is, 

 that they are used for progression, or it may be to 

 assist the animal in throwing off its arms, a practice 

 the Brittle Stars have in common with many of the 

 Echinodermata, to the great anroyauce of the col- 

 lector. Professor E. Porbes gives the following 

 graphic description of the process : — 



" The common Brittle Star often congregates in 

 great numbers on the edge of scallop banks, and I 

 have seen a large dredge come up completely filled 

 with them, a most curious sight ; for when the dredge 

 was emptied, the little creatures writhing with the 

 strangest contortions, crept about in all directions, 

 often flinging their arms in broken pieces around 

 them." Describing the Lingthorn, Luidia fragi- 



