CHAETOGNATHA. A 
peculiarly prominent manner, for there are few other groups of animals of which as many 
as ten or even more species of one genus are found together in exactly the same environment. 
The question of the nature and extent of the variation in each species is also of 
interest, for there are indications that the characters commonly relied upon as distinctive 
may sometimes be very untrustworthy. It has been pointed out how in some species local 
races exist with slightly different characters, as in the case of Sagitta serratodentata and 
S. hispida, but some species are markedly variable in the same locality. For example, 
besides the variation in number of the teeth in S. tricuspidata mentioned above, it was 
found that while the typical number of hooks is 8, one specimen had only 4, another 
5, and others 7 on each side. 
I observed a more remarkable case of this at Naples, which possibly indicates that 
the hooks are lost to some extent at maturity. The species in question was Sagitta lyra, 
which Grassi in his monograph (Fauna et Flora des Golfes von Neapel; I Chetognati) 
describes as being very rarely found sexually mature, while immature specimens are 
comparatively common. At Naples during the early spring of 1901 immature specimens 
were frequent in the “Auftrieb” from no great depth, and many were of considerable size, 
eg. as much as 28mm. with only most minute rudiments of ovaries and_ testes, and no 
trace of genital ducts. In April, however, a number of specimens were caught in the 
neighbourhood of Capri at depths of 400 and 1000 metres, and these were mostly sexually 
mature. Those from the greater depth were remarkable in that a large proportion had 
only three hooks on each side instead of seven, although others were nearly or quite mature 
with the normal seven hooks. These examples had otherwise all the characters of S. lyra, 
except that the head was perhaps shorter and broader than usual. 
These facts seem to indicate that either there are two closely allied species or varieties 
included under the name S. lyra, or that when maturity is reached, four out of the seven 
hooks on each side are, in some cases at least, lost. It also seems to suggest that when 
mature this species migrates to a much greater depth, for no fully adult specimens were 
taken at the surface. Grassi did most of his work at Messina, where the currents bring 
up to the surface animals which normally live in deep water, and this probably accounts for 
his finding occasional adult specimens. 
With regard to variation in different localities, it appears that most of the widely 
distributed species differ to some extent in widely separated areas, as is mentioned above 
in respect to the teeth. It is interesting to note that in all cases in the present collection, 
where the teeth differed in number from those of the European variety, they were more 
numerous, so that the average number of teeth in specimens from the Indian Ocean is 
considerably greater than from Europe. Another character which distinguishes the Eastern 
Sagittas from the European as a whole is the frequency of intestinal diverticula in the neck; 
these are found only in one European species, but five, or more than one-third of the whole, 
of those from the Maldives possess them. The same fact has been noticed by Conant with 
regard to the American Sagittas, a large proportion of which have the diverticula. 
The fact that so many species should be common to the Eastern coast of America and 
to the Indian Ocean, although not found between, is remarkable, but is probably explained 
by the absence of complete lists from the Southern Atlantic. When the latter area has 
been more carefully examined, it will probably be seen that this apparent discontinuous 
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