1210 



SCANDIXAVIAX FISHES. 



PISCES LEPTOCARDII. 



Fishes whose unaltered notochord extends further forward than the spinal cord, which is 



of uniform thickness and without true brain. No head proper; no true heart. Respiratory 



apparatus a large, retiform branchial basket. 



c h. 



Fig-. 364. Lanrelet (BrancMostoma tanceolatum), seen from the left, with the skin and muscles removed from this side. About 3 times 



nat. size, .\fter VoGT and Ycng. 

 a, dorsal, b, ventral fin-ray growths; c, organs of generation; d, branchial basket; e, anal aperture; /, notochord; g, spinal cord; /), intestinal 

 appendage (hepatic c«cmn), situated on the right side, seen through the branchial basket; >', intestine; k, mouth; ', great lateral muscle; 



m, rostral fin; ?), caudal fin; o, abdominal pore. 



Here it may be questioned even more than in the 

 jjreceding order whether these creatures are vertebrates 

 or invertebrates. It is not only the low rank occupied 

 by this order in comparison with the preceding ones; 

 we shall besides find here an asymmetry that strongly 

 reminds us of certain invertebrates. 



Johannes Muller interpreted this order as a se- 

 parate subclass among fishes — I)ut so he also regarded 

 the Cyclostomes and Elasmobranchs ■ — and gave it the 

 name of Leptocardii" . Under the name of Cirrosfomi 

 the order was coupled by (^wen' together with the 

 Cyclostomes in a subclass, TJermopteri (with fins in the 

 form of dermal folds). Others have gone still further 

 and regarded the order as a separate class among the 

 vertebrates. So the Leptocnrdii, under the same name, 

 were conceived by Gill"; and E. Hkckel called them* 

 Vertehrata Acrania. Othei's' have indeed considered 



these creatures to be the lowest vertebrates, but have 

 assumed that thej' attained this position by a retro- 

 grade develo])ment (degeneration) from better equipped 

 vertebrates, which have now disappeared. Others again 

 have ranged them beside the Tiinicata, and regarded 

 them as one of the types for the predecessors of the 

 prtjtovertebrates (the supposed ancestors of tlie verteb- 

 rates). Thus Balfour called them-', together with the 

 Tunicates, Protochnrdata (primordial forms among the 

 animals furnished with permanent notochord), and con- 

 ferred upon them the S])ecial uiinie of CephalocJiorda 

 (with notochord in the head), a. reference to the for- 

 ward extension of the notochord in that part which 

 answers to the head of the vertebrates, as distinguished 

 from the Tunicates (Urochorda), which have a notochord 

 only in the tails of the larvje. The significance of 

 these divergent opinions is best a]ipreci;ited after we 



" Ob. d. Bau u. d. Grenccn dcr Oanoiden, Abh. Akad. Wiss. Berl., 1844. Physik. .-ibh., p. 204, lerctOQ, thin and /MQdUt, heart. 



' Comp. Anat., Phys. Vertebr., vol. I, p. 7. 



« Arr. Fam. Fish., Smiths. Misc. Coll., No. 247, p. IX. 



'' Gener. Morphol. Organ., Bd 2, p. CXIX. 



' DOHRN, Ursp. Wirbelth., Leipz. 1875, pp. 51 — 55. 



^ Comp. Embrtjnl., vol. II, p. 271. 



