AN AUSTRALIAN OHAETOGNATH — JOHNSTON. 253 



The last specimen is the most typical and has been selected as 

 the type, the description referring mainly to it. 



The head is fairly prominent, its breadth being about 1/8 mm. 

 and its length one millimetre. The jaws are set fanwise, the 

 animal being in a state of medium contraction. There is a neck 

 constriction present, the breadth here being one mm. From the 

 point the body widens rapidly to 1*2 mm. and then gradually to 

 2'7 mm. in the region of the ventral ganglion, finally reaching 

 32 mm. at about the middle of the body. The breadth is fairly 

 well maintained almost to tail septum, where there is a sudden 

 narrowing to 1*5 mm. The tail is four mm. long, thus being 

 one-sixth of the total length of the animal (sixteen to eighteen 

 per cent.). The extremity is rounded. There is no collarette 

 present. The lateral fields are rather wide, and the longitudinal 

 muscles moderately developed. Consequently the worm is trans- 

 parent and flabby (Plate Ixxi., fig.l). 



There are two pairs of fins, the anterior being carried out of 

 their normal position in most specimens, probably the result of 

 preservation in alcohol. The relative position and size of the 

 anterior and posterior fins have been determined by examining 

 serial sections kindly lent by Professor Haswell. The two pairs 

 are approximately equal in length. The anterior is perhaps 

 slightly longer and extends from just in front of the ventral 

 ganglion backwards for about five mm., its greatest width being 

 at some distance behind the ganglion. It is not quite as wide as 

 the posterior fin. Between the two tins on the same side there is 

 a considerable interval amounting to about eight per cent, of the 

 total length of the worm. The posterior fin is somewhat trian- 

 gular, the apex being rounded. The greatest width is just 

 behind the tail septum. The anterior end does not extend as 

 far forwards as the front of the ovary in some specimens, while 

 in others it reaches beyond it. The posterior extremity of this 

 fin is 1*3 mm. distant from the vesicula seniinalis. The tail fin 

 is bilobed and prominent. It does not extend as far forwards as 

 the seminal vesicles. Rays are present in all fins. 



A characteristic feature of the Chaetognatha is the possession 

 of jaws, the number varying within the species. If the table be 

 referred to, it will be noticed that there are nine pairs in every 

 specimen examined, though the animals vary in size from twelve 

 to twenty-four millimetres. The presence of nine pairs may be 

 specifically constant. They are rather strongly curved, the back 

 of the " point" of the jaw being a little more bent than the back 

 of the " shaft." The inner edges of point and shaft are evenly 

 curved. The jaws of Sagitta furcata, Steinhaus, and S. enfiala, 



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