THE CH^TOGXATHA. 57 



It may, then, be asked, what is left by which to distinguish the species of Chpetognatlia ? 

 The general outline of the l)ody counts for something ; that is to say, the propovtion of 

 head to body, the i^osition of the tliickest part of the l)ody, the ratio between the lengths 

 of the postanal region and the trunk. Of these only the last one can, as a rule, be 

 safely expressed in measurements, as the proportions of head and body vary so much 

 with the extent of the death-contraction. Other fairly fixed characters are the presence 

 or ahsence of a constriction between head and body to form a -'neck"; a lateral 

 expansion of the ectoderm at this point * ; a sudden diminution of the body-diameter at 

 the anus as contrasted Avith a gradual tapering, or a marked constriction at the 

 septum f. 



The consistency of the body is of some value: those species -with comparatively large 

 longitudinal muscles and small muscle-free lateral fields (e. g., bipimctata) have a firmer 

 consistency, and in formalin a more ofiaque appearance, than species with comparatively 

 poor musculature and large lateral fields, such as hexaptera; these seem Habby and 

 transparent in formalin, and often have a wrinkled appearance, if not very carefully 

 preserved when ali^e. 



As regards tlie lateral lins, if well preserved, a comparison of their width with that of 

 the body, and a comjiarison of the widths and lengths of the two pairs, are more safely 

 expressed l)y camera drawings than by measurements ; their extension Ibrwards as 

 tested by the ventral sjangliou, backwards as tested by the vesiculse seminales, centrally 

 as regards the anal septum, are of great value. The tail-fin is less instructive ; its 

 extension forwards as regards the vesiculfe seminales is useful, l)ut its outline is very 

 readily damaged, and a truncate fin easily splits into an appai'ently bilobed one, or wears 

 into an ensiform shape. Too much stress must not be laid, except in perfectly 

 preserved material, on the trifmgular or elliptical outline of the posterior fins: it is 

 quite easy to find specimens {e. g., of f areata) witli an almost triangular fin on one side 

 which had been rubbed into an ellipse on the other. 



1' he most stable 2«^i"ts of a Cha?tos'nath are formed by the cephalic armature — the 

 javvs {cin-hi, uiicini) and teeth J. These are often the onli/ characters by which to 

 identify a badly preserved specimen, l)ut they must be used with care, since, as I shall 

 show later in dealing with the various species captured, they are liable to considerable 

 variation. Still, allowing a considerable latitude for these variations, their mere 

 numbers afford a fairly accurate criterion when taken in conjunction with the characters 

 already cited. If to these be added a detailed study of the jaw-structure, such as 

 Dr. Krurabach § has recently worked out for several sjiecies, the observer may feel 



* For this, which lias heen termed a " iieck-fiii," I have used the name " collarette.' 



t Dr. Krurabach writes that he has worked out the form of the taii-scgmeut as a diagnostic character for 

 Mediterranean species. 



t The counting of these is often troublesome. I use two short slips of glass cut from a slide, lay one across the 

 dorsal surface of the specimen just behind the head, and then, bending the head upwards and backwards with a 

 needle, prevent it from falling forwards again h\ pushing the second glass slip up against it ; aud drop a cover-glass 

 on the whole. This enables one to study the armature from the anterior end, and generally to count the teeth and 

 jaws readily. 



§ Th. Krumbach: " Ueber die Greifhaken der Chiitognathen," Zool. Jahrb. (Abth. Syst. u. s. w.) xviii. 579 

 (190;3). 



10* 



