4 

 The extent and nature of the contraction of the head in death makes a considerable 

 difference to the look of the anterior end of a Chaetognath. Three main conditions are 

 distinguishable : 



(i) the jaws are thrown outwards, radiating from the head, divergent from each other, like 

 the sticks of a curved fan; the head is broad and long; the condition is that of extreme 

 expansion : 



(2) the jaws are turned inwards, with the tips directed forwards, so that their long axis is 

 nearly parallel to the long axis of the body; they are closely pressed together, and are 

 often covered by the prepuce; the head is long and narrow, the condition is that of 

 normal contraction : 



(3) the jaws are turned yet further inwards, so that the tips meet or even pass one another 

 across the mouth: the jaws are then nearly at right angles to the long axis of the body; 

 the head is short and broad; the condition is that of extreme contraction, and may often 

 be seen in specimens killed in the act of swallowing their prey. 



Between these three all possible gradations are found, and the alterations produced in 

 the shape of the head, the collarette, and the corona are very great; the comparative position 

 of the rows of teeth, and the angle of inclination of the individual teeth, become greatly altered. 



The statement in Biscayan Plankton relative to the diagnostic value of the diverticula 

 on the alimentary canal (p. 56) requires modification to this extent: — that the possibility of 

 recognising these structures, by focussing into a transparent specimen, depends largely upon 

 the state of contraction and expansion of the head and neck. In fig. 100 is represented, 

 diagrammatically over a camera lucida outline, the arrangement of these structures, as seen in 

 a horizontal longitudinal section of a transparent specimen of ferox, in which they were 

 conspicuous from the outside. But similarly transparent specimens, in which they were quite 

 unrecognisable beforehand, gave the relations shown in fig. 10 1 (horizontal longitudinal) and 

 102 (transverse section). It will be obvious from these figures that the oesophagus (presumably 

 an ectodermal stomodaeum) grows backwards in a dorsal and a ventral prolongation, and that 

 the intestine (presumably an endodermal mesenteron) grows forwards in two lateral prolongations, 

 which, on the fusion of the two parts, give rise to the diverticula. The epithelia of the 

 (presumed) ectoderm and endoderm are of quite distinct characters. If the head is expanded, 

 this whole region is pulled forward, and the diverticula become conspicuous; in strong contraction, 

 it goes backwards, and the structures become so compressed together as to be distinguishable 

 no longer from outside. 



I have already contended in 'Biscayan Plankton' that the projection, size, and shape 

 of the vesiculae seminales depend on the sexual condition of the individual at the moment of 

 capture, and cannot be utilised for specific diagnosis. In illustration of this I have here drawn 

 by camera lucida (fig. 103) the outlines of the posterior ends of four specimens of S. ferox, 

 which measured respectively 11, 15, 16, 17 mm. of total length. The two longest had developed 

 the 'callotta splendente' of Grassi; in the shortest, hardly any trace of the vesiculae was apparent. 

 A similar criticism may be applied to the use of the extension of the ovaries in diagnosis; 



