LAIiVAL AND TOST-LAUVAL KISJIKS—REOAN. 151 



out info filaments wliicli may l)e used as feelers, perhaps to find (nod at night. 

 Wlu'n tiie pelvic tins ch'velop precociously they also mav take the tuim (if spines 

 (e.g., Thi/rxiti's) ^)\■ oj' long filaments (e.g., 'rnidii/ptrnis) ; Imt in othci- cases tliey grow 

 out into large fins with the rays fully connected liy memhivnie (e.g., Pnqctopftift 

 i)iiir/;i/i/er/is, p. ]:',i'. I'l. Ill, figs. l-;5) : presumalily siidi tins wonid he used to liclp tlic 

 pectoi'als in halancing and to prevent the fisli fi'oni sinking. 



it seems that in most fishes tlie fins (h'Nclop in the same order and tliat the 

 development of one or more fins out of tliciiturn. ])recociouslv, may he for pmiioses 

 either of fiotation, lialance, defence, or perhaps nocturnal feeding. 



4. L'HAltAUTElUSTIU FEATURES UF FEEAGIC LARVAE. 



Absciire af in'ccssuri/ (iiy/iiis of rcsj/initioii mid of ndheslvi' on/ans, — Larval 

 structures that occur in more than one group of fresh-water fishes are external gills 

 and adhe.sive organs : the latter enable them to remain in the place selected by the 

 parents until tlie yolk is absorbed, and external gills are advantageous when the water 

 is limited in quantity or deficient in oxygen : that tliese structures are not found in 

 pelagic marine larvae is not surprising. 



fiiris/bllifi/. — All pelagic larvae are transparent, and in some groups the larval 

 stage is prolonged until a considerable size is readied ; these large larvae remain 

 transparent owing to the strong compression of their l)odies and the looseness of their 

 tissues, and the change into the less compressed and more compact young fish is 

 accompanied l)y a shrinkage. This type of development is characteristic of tlie Apodes 

 (p. f40, n. ^'^, figs. 5-7), but it occurs also in the Elopidae and Albulidae. 



Jjiioj/anci/ and bidniice. — It has been suggested that the dorsal sinus of the 

 Myctophidae (pp. 127, 138, PL VI) may serve as a fioat, and it seems likely that the 

 large pelvic fins of the larval Chaenichthyidae, especially Payetoims (p. 132, 1*1. Ill, 

 figs. 1-3), may be sjiread to prevent the fish sinking. 



Generally the vertical fin acts as a keel, and balance is maintained bv movements 

 of the pectorals; jiossilih' the protrusion of tlic to'ininal ])art of tlie gut in man\' 

 larvae may be connected with balance ; in those descril)ed as Sti/lophtliulimis the 

 protruded portion may be quite long, but it is difficult to understand whv tliis feature 

 should l)e so exaggerated as it is in Styhi]>hfhalmus vnicrcnfcron (p. 13G, I'l. \, fio-. i). 

 Tlie terminal part of the gut, with its Itasal support, forms a long appendage; this 

 would, presumably, keep tlie fish steady, and make it difH(;ult for it to tnrn omt on its 

 back, but would retard its progress if it attempted to swim : in fact, one may suppose 

 that the chief effect of flexions of the tail would be to lotate the fish, tlie anal 

 appendage serving as a fulcrum. 



Sense on/ans. — Certain fin-rays may be foimcd precociously and grow oiii into 

 long filaments ; these may be used as feelers, perhaps to fiml food at night. In all 

 larvae tlic auditorx'. optic and olfactoi'X' organs ap])eai' to be well dc\clopcd. but it is 



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