5 



in the two longest of the above four specimens they reached to the neck of the animal, in 

 the specimen of 15 mm. only to the ganglion; in that of 1 1 mm. they were mere rudiments. 



In the extension of the posterior fin backwards there lies a trap ; in young specimens 

 of some Chaetognaths, this fin appears to stop at some distance from the undeveloped vesiculae ; 

 but as male maturity approaches, the vesiculae lengthen and swell till they touch the fin. 

 Mutatis mutandis, the same is applicable to the tail fin. 



The generic nomenclature used here is that of Langerhans. 



As no less than nineteen out of the twenty-four species of Chaetognatha which appear 

 to the writer to be 'valid', were found either in the Sibogan or the Biscayan collections, an 

 attempt has been made at a faunistic and systematic revision of the whole group. 



Every one who has attempted to identify Chaetognatha in recent years must have been 

 painfully aware of a number of very similar forms among the recorded 'species', which for the 

 most part have been but cursorily described. To attempt to reduce, the number of species in 

 such a case is an ungracious task, and, if the original specimens are not available for comparison, 

 lays the attempt open to an unsympathetic criticism. Nevertheless, it has been made, in the 

 hope that the task of future workers in this group may thereby be lightened. At least it can 

 safely be maintained that, if a (perhaps 'good') species has been so inadequately characterised 

 that it is possible to confuse it, and on paper to combine it, with another and better known 

 species, its present place should certainly be in the synonymy rather than in the systematic. 



