lIEMIBKAXrilS. 



6;^,5 



PHYSOCLYSTI HEMIBEANCHII. 



r/n/siii'liisfs iritit free jiiii'lxnics and pcrfiiiafed (jillf, but irith the liraiicl/ial arrlies more or less imperfect and 



iritJi J(ir<ie and erfernalli/ rinUtlc iutcrrlarieles. 



Here, as in the case of tlie Plectogiiates, the sy- 

 stematic difference fVoiu the other Physoclysts is not at 

 all too i.n-eat to admit of the arrangement of these fishes 

 in a series of families corresponding to the series already 

 iidopted among the Physoclysts and ranged beside these 

 series. This was also the approximate rank assigned to 

 them by the writer who first treated of the combination 

 of these fishes into a systematic whole: in 1861, in his 

 arrangement of the Acanthopterygians", GOnther adopted 

 a twelfth di\ision, Acantliopterygii gasterosteiformes, in- 

 clnding the Sticklebacks and Flute-mouths with their 

 nearest relatives, and a thirteenth division, Acaidhopte- 

 ri/f/ii cenfristiforiiies, t-ontaiuing the family of the Trum- 

 pet-fishes. After Parkkr's* deraonsti-ation in 1868 of 

 the morphological significance of the interclavicles of 

 the Sticklebacks as traces of the (ianoid t\'iie, (Jope" 

 united GrNTHEu's two divisi<_)ns into nn order Ifemi- 

 hraiuliii, ■■connecting the Lojdiohraiicliil with ordinary 

 fishes. ' 



All these fishes have abdominnl ventral tins, and 

 are thus ranged comparatively low in the scale of deve- 

 lopment of the Phvsoclysts. Theii- ])rincipal character 

 also lies, as we have shown ahove, in the circumstaTice 

 that several of the bones which in more typical Phy- 

 soclysts entirely partake in the structure of the endo- 

 skeleton, are here wholly or ]),-irtlv dermal, appearing 

 in the form of growths Ijelonging to the exoskeleton. 

 This structural feature is fairly common among fishes. 

 Scales, differing usually in size, shape, or thickness from 

 the other scales of the body, are in one fish the pre- 

 cui'sors of ;i still further alteration in a kindred form, 

 a difference which, during the development of the form- 

 series, leads to the result that the modified scales creep, 

 so to speak, into the body and join themselves to parts 

 of the endoskeleton, and finally, when the transforma- 

 tion has reached its highest point, lie like covering 

 (membrane) l)ones outside other bones, oi- even enter in 



the form of independent bones into the endoskeleton. 

 A great portion of the skeleton of the Vertebrates has 

 originated in this manner. The most beautiful and most 

 perfect example of tins is given by the luiniaii clavicles, 

 the development of which has l)een traced b^- Pahker 

 (1. c.) from their first origin, at which period they are 

 mor|jhologically analogous to two lateral plates in the 

 Sturgeons or the Cuirassed Siluroids''. The intercla- 

 vicles which we have above remarked in certain Plecto- 

 gnates, are of similar origin. They occur, it is true, 

 in several other Physoclysts as well, but in an extre- 

 mely reduced form and as parts of the endoskeleton. 



Hei"e, in the Henilhrandiii, as well as in the Lopho- 

 hranchii, they still preserve distinctly the character of 

 dermal plates. Thus, in the Three-spined Stickleback 

 (fig. 1.5(5, (') for example, they have the form of two 

 parallel, ventral scutes, contiguous in front, separated 

 l)ehind, on the outside gi'anulated and striated, like the 

 other dermal plates of the body, on the inside support- 

 ing, in front, the lower ends of the clavicles, and be- 

 hind, the lower margins of the coracoid bones or more 

 correctly, according to Parker, of their covering bones, 

 the procoracoids, which are also said to be of dermal 

 origin. 



The same relative position to the rest of the 

 skeleton is here occupied by the pelvic bones or rather, 

 according to the same interpretation, by the covering 

 bones of the pelvic liones proper. These bones lie in 

 the Sticklebacks immediately behind the interclavicles, 

 or even project some distance between them; and in the 

 Three-spined Stickleback (fig. l.')7, A. rs) they are 

 marked by the large, ascending process that meets the 

 lateral plates of the body in the skin. The character 

 which in (Joi'e has been chosen to g^ive the Hemihranclm 

 their name, lies in the absence or persistent cartilagi- 

 nous structure of one or more of the upper parts of 

 the branchial arches (the epil>r;inchial and upper pha- 



" iSnstcm. Synops. Fam. Acaidhopt. Fisli., Cat. Brit. Mns.. Fish., p. VIII. 



* Uay. Soc, Shoulder Girdle and Sternum, p. 40. 



' Ichthi/ologi/ of the Lesser Antilles, Trans, .\iner. Pliil. Soc, Pliilarl., ii. ser., vol. XIV. pp. 45t; ct 457. 



'' Cf. Smitt, rjr lie hdr/rr djurens utvecklingsliistoria, p. 218. 



