1210 
SCANDINAVIAN 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. 
Fig. 364. Lancelet ( Branchiqstoma lanceolatum ), seen from the left, with the skin and muscles removed from this side. About 3 times 
nat. size. After Vogt and Yong. 
a , dorsal, b, ventral fin-ray growths; c, organs of generation; d, branchial basket; e, anal aperture; /, notochord; g , spinal cord; h , intestinal 
appendage (hepatic caecum), situated on the right side, seen through the branchial basket; i, intestine; k, mouth; l, great lateral muscle; 
m, rostral fin; n, caudal fin; o, abdominal pore. 
Here it may be questioned even more than in the 
preceding 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 — but so he also regarded 
the Cyclostomes and Elasmobranchs — and gave it the 
name of Leptocardii 11 . Under the name of Cirrostomi 
the order was coupled by Owen * 6 together with the 
Cyclostomes in a subclass, Dermopteri (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 Leptocardii , under the same name, 
were conceived by Gill c ; and E. Heckel called them* 6 
Vertebrata Acrania. Others e have indeed considered 
these creatures to be the lowest vertebrates, but have 
assumed that they attained this position by a retro- 
grade development (degeneration) from better equipped 
vertebrates, which have now disappeared. Others again 
have ranged them beside the Tunicata, and regarded 
them as one of the types for the predecessors of the 
protovertebrates (the supposed ancestors of the verteb- 
rates). Thus Balfour called them 7 , together with the 
Tunicates, Proto chord at a (primordial forms among the 
animals furnished with permanent notochord), and con- 
ferred upon them the special name of Cephalochorda 
(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 larvae. The significance of 
these divergent opinions is best appreciated after we 
a Ub. d. Bau u. d. Qrenzen der Ganoiden, Abh. Akad. Wiss. Berk, 1844, Physik. Abh., p. 204, Xsmog, thin and yMgdla, heart. 
6 Comp. Anat ., Phys. Vertebr., vol. I, p. 7. 
c Arr. Fam. Fish., Smiths. Misc. Coll., No. 247, p. IX. 
d Gener. Morphol. Organ., Bd 2, p. CXIX. 
e Dohrn, Ursp. Wirbelth., Leipz. 1875, pp. 51 — 55. 
f Comp. Embry ol., vol. II, p. 271. 
