HARDWICKE'S SCIENCE-GOSSIP. 



stant. Their use is to assist the odontophore in 

 triturating the food. The whole jaw assumes, however, 

 a characteristic form peculiar to its own genera. In 

 a common black slug {Arion aier) of this neighbour- 

 hood, I found a semi-oval jaw, with numerous ribs of 

 varying breadths, smooth on the concave and crenu- 

 lated on the outer aspect, and barely projecting beyond 

 the edge of the base (fig. i). Whilst in a jaw of 

 Succinea pittris, from the towing-path of the canal in 

 the centre of Leeds, we have a central tooth and a 

 quadrate plate behind (fig. 2). 



Very little seems to have been done in determining 

 the composition of Molluscan jaws, and books on 

 zoology, microscopy, anatomy and conchology, 

 seem content to share in the stereotyped and often 



ric, sulphuric, and acetic acids, potassic hydrate, 

 Schultze's syrup, &c, I found that they were chitinous 

 and not corneous or horny. 



Keratine, the basis of horn, hairs, feathers, &c, has 

 a different chemical composition to chitin, and 

 behaves unlike it under the same tests. The methods 

 pursued in determining chitin and keratine are too 

 familiar to be more than alluded to here. Cartilage, 

 as is well known, is only found amongst invertebrates 

 in Cephalopoda. The nucleated appearance of car- 

 tilage and the laminar structure of chitin under the 

 microscope are so dissimilar, that the cartilaginous 

 nature of jaws must have been a guess and never 

 tested. That they are not calcareous needs no con- 

 tradiction, although salts of lime are deposited on 



Fig. 1.— Jaw of Arion aler, Leeds. 



Fig. 2. — Succiiica ptttris, Leeds. 





Figs. 3-5. — Helve netnoralis, Whitby. 



synonymous terms, composition corneus, horny, 

 cartilaginous, calcareous. 



Through the microscope they have every appear- 

 ance of being chitin (C 15 , H 26 , N 2 , 22 ) a 

 substance particularly familiar to the worker with 

 this instrument. Chitin might aptly be called inver- 

 tebrate bone, for it enters into the composition of the 

 endo- and exo-skeleton, locomotory (aerial, terrestrial, 

 and aquatic), digestive, generative, and respiratory 

 organs, &c, of various members of that great division. 

 The Molluscan jaw and the serrated teeth of leeches 

 (Hirudo), are but modifications of the same substance 

 to suit divergent habits. On testing chemically the 

 jaws of Helix ncmoralis and hortensis and portions 

 of the gladius of Loligo vulgaris with hydrochlo- 



them occasionally. In my cabinet I have a jaw of 

 Helix arbustorum on which are three star-like clusters 

 of crystals, which polarise beautifully. 



Chitin, being a derivative product, does not involve 

 the consequent destruction of its constructive cells ; 

 whilst the opposite obtains in keratinous substances, 

 their original cells having been transformed into 

 horny matter. In vol. iii. pt. 8 of the "Journal of 

 Conchology," the editor in an article on Helix arbus- 

 torum takes us a step further. He states that the jaw 

 "is composed of indurated or hardened mucus and 

 is of a horn colour ; the minute sculpture is formed of 

 longitudinal wavy lines which follow the exterior 

 outline." These lines of sculpture are the lines of 

 deposition of the chitin ; the alternate hard and soft 



