126 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



Though the junction of these jaws is pretty close above and below, 

 they seera as it were to strangle the Radula, and make the teeth 

 project out in a kind of point or bunch. One can easily understand 

 when looking at this instrument, how it is that the Siphonostomata 

 are able to bore holes in shells of such an exactly rounded shape. 

 The appearance of the buccal mass is very much in shape like 

 Teredo navaJis, L., having the same broad, blunt, conical 

 outline. All the Siphonostomata are carnivorous feeders it is 

 said. As a rule the Radula is very short, and is composed of a 

 short series of long hooks with a sharp blade and a broad curve, 

 something like a sickle. The central teeth are broad, simple 

 chisel-like forms. (See Fig. 4.) 



Every one who has examined these animals must have noticed 

 the redness of the termination of the tube. If this is inspected 

 carefully, it will be found that there are two distinct muscular 

 bands, crossing each other, both tinged with red blood. From the 

 back of the buccal mass the oesophagus lies loosely in the tube, 

 being fastened underneath by a narrow series of muscular bands 

 which secure it, but give the greatest freedom of movement. 



Crepidula aeuleata, Gml., is a small shell which is found in 

 Port Jackson, and I believe in most temperate seas of the World, 

 and strangely enough, always as a kind of tenant in the shell- 

 mouth of a Triton. It is one of the Siphonostomata, with a shell 

 of inconspicuous color but peculiar slipper shape. The most of 

 the animal is located under the partition which extends half-way 

 across the shell, giving it the appearance of a minute slipper. 

 The animal is beautifully mottled brown and pale yellow over the 

 region of the viscera, and is quite visible through the septum 

 which is transparent. The siphon projects out through this and 

 is conspicuously red. The Radula is of the usual type, such as I 

 have described above ; but there is a peculiarity at the base of 

 the sickle-like teeth which I have not observed in other species. 

 At the curve of the shaft there is a row of eight or nine tubercles 

 decreasing in size from the centre. 



I may here mention a circumstance connected with the 

 circulation, which I could not more conveniently introduce 

 elsewhere in this essay. I shall have occasion to describe 

 subsequently the mode in which the shell-structure is permeated 

 by perforations and nerve fibres, to an extent which almost 

 destroys our previously received ideas of its compactness and 

 solidity. In some of the sections I have observed small blood- 

 vessels permeating the shell-structure as well. These vessels are 

 of extreme tenuity, not more than -^hru of an inch in diameter ; 

 but the most singular fact connected with them is, that something 

 like valves are observable at regular intervals all through the 

 length of the transparent tube. I am not aware whether valves 



