STRUCTURE OF THE CHONDRACANTHUS LOPHII, 69 



divided by a slight constriction into two parts — a terminal hook and a basal 

 shorter portion. It is attached to the ventral aspect of the animal by a very 

 peculiar arrangement, which is worthy of special notice. On each side of the 

 middle line of the ventral aspect at this spot is a triangular stage or frame, 

 the sides of which are formed of bands of chitin, and the inner angles of which 

 meet in the mesial line. To the whole of the outer, and the adjacent parts of 

 the anterior and posterior sides of this frame, the basal segment of the great 

 hook is closely connected ; but to the remaining portions of these sides it is 

 attached by a loose piece of integument, so that, when the hook moves inwards 

 to the mesial line, i. e., when the inner angle of its basal segment is depressed, 

 the integument at this spot has a plicated appearance ; on the other hand, when 

 the hook moves outwards, it is put on the stretch. The movements of the hook 

 are regulated by transversely striped muscles : the one attached to the internal 

 angle of the basal segment — the flexor muscle — being more strongly developed 

 than the external, or extensor, muscle. The outer coat of the hook is hard, and 

 even brittle, from the quantity of chitin substance which enters into its com- 

 position. 



Situated behind the hooks, and at a short interval from them, are three pairs 

 of well-marked foot-jaws (fig. 5, c). Immediately in front of the bases of the 

 first pair of jaws, and consequently placed between them and the hooks, is a 

 transversely oval opening, the mouth (tig. 5, s). It appears to a great extent to 

 be overlapped by a fold of skin, which passes backwards from the framework of 

 the hooks. When the hooks are approximated to the jaws, this fold of skin 

 completely conceals the oral aperture. Rathke states that the mouth is situated 

 upon a small snout-like elevation. In our various specimens we were unable 

 to see this proboscis, which may probably be accounted for by the fact, that 

 when we proceeded to examine into the anatomy of this part of the animal, it 

 had been for some time in spirit, the snout perhaps only being projected at the 

 time when the animal is sucking. Van Beneden and Glaus, again, appear to 

 think that the mouth is surrounded by the jaws ; but repeated observations have 

 convinced us that this opening is situated in front of the first pair, though perhaps 

 slightly between their bases. 



The three pairs of jaws are situated a little to the outer side of the middle 

 line, one behind the other, in regular order. The first and second pair are so 

 close together that the first is partially overlapped by the second. The third, on 

 the other hand, lies at a greater distance from the other two, and somewhat 

 further removed from the middle line. Each of the first pair (Mandibles, 

 Rathke) consists of two segments, a basal, short and thick, and a terminal, 

 on which the cutting apparatus is placed (fig. 6, c). The terminal part is 

 formed of two chitinic plates, which are blended together by their anterior 

 margins; the posterior margins are separated by a longitudinal furrow, and 



