266 
T. Davidson — What is a Brachiopod ? 
affinities of tlie Bracliiopoda liave been kindly communicated to me 
by Prof. W. King, Sc.D. : — 
“ The group Palliobranchiata emhraces fonns formed of two types of Organiza- 
tion. The absence of an anal vent in the Clistenterata makes them inferior to 
the aniferous Tretenterates. In determining the affinities of the group first 
tlioughts should be directed to the simplest of these divisions : but a difficultv 
arises ; for, so far as is known, the second and highest division was the first that 
made its appearance. This, however, may be got over on the supposition that the 
Tretenterates became degraded into the Clistenterates, — such a metasmotosis is not 
unknown ; for example, the ventless Ophiurids are outcomes of aniferous pluteiform 
larvae, and probably a similar metasmotosis takes place in certain ventless asteroids 
( Astropecten , Zuidia, etc.). Adopting this supposition, the Clistenterates may be 
dismissed from further consideration. I shall, therefore, regard the Tretenterates 
as the initial or older type. The Cambrian System is not only the first in whieh 
indisputable organic remains oceur, but it is the one in which the Palliobranchs 
make their first appearance, and, as far as is known at present, they appear to be 
exclusively Tretenterates ; the genera, with perhaps one exception 1 ( Orthts Hicksii), 
being all more or less related to Lingula and IHscina. Associated with these 
Cambrian Tretenterata there are remains of organisms belonging to other groups 
besides the Palliobranchs. According to the doctrine of Chronogenesis, the natural 
affinities of any group of organisms can only be determined by its structural cliarac- 
ters being considered in connexion with those of other groups of contemporaneous 
(geological) origin , 1 2 I shall attempt to give a table of the Cambrian fossils of 
interest in connexion with the present subject, separated into their respective groups, 
which I have drawn up from Hicks’s momoir on the Tremadoc rocksinthe neighbour- 
hood of St. Davids, in South Wales : 3 — 
Protospongia 
Oldhamia j 
Theca 
Lingulella , etc 
Palasterina 
Histioderma , Scolithus 
Paradoxides, Agnostus , etc. 
Spongida. 
f Hydrozoa. 
? Polyzoa. 
? Pteropoda. 
Palliobranchiata. 
Asteridia and Ceptoidia. 
Annelida. 
Crustacea. 
Of these groups the Palliobranchs have often been associated with Polyzoa ; but 
lately Morse has endeavourcd to show that they are more closely related to one of 
the others, Annelida. There is no doubt he has succeeded in adducing certain points 
in liis favour ; but there are so many dissimilarities between the annelids and the 
palliobranchs that it is scarcely to be expected the polyzoonal alliance will be 
abandoned by those who have contended for it, especially since the discovery of 
Rhabdopleura, a marine form of Hippocrepian polyzoa . 4 * 6 There is another group of 
organisms, also occurring in Cambrian rocks, which does not appear to have attracted 
mucli notice in connexion with the subject in band — I allude to the Asteridia. The 
late Johannes Müller showed that in the larval state certain star-fishes have their 
form completely different from what it is in the adult state, changing from bilateral 
to a radical character. This is especially the case with the larval star-fish ( Bipin - 
naria asterigera ), which possesses features strongly reminding one of the structure of 
Lingula. It has a large peduncular appendage at the posterior end of the body ; it is 
furnished with a pair of tentaculiferous arms bilaterally arranged, with the tentacles 
ciliated; its mouth is situated between and at the base of the arms; it has an 
intestine, which is doubled back on itself, and terminates at one side as an open gut . 3 
This seems so very like the general structural plan of Lingula that I prefer asso- 
1 An Orthis (possibly a Clistenterate) has been found in the Menevian group, but to whatever 
division it may belong does not matter, as Lingulella oceurs at the base of the Cambrians. 
* See a memoir by Prof. King, entitled, “ An Attempt to Classify the Tetrabranchiate 
Cephalopods.” Annals of Natural History, in 1845. 
s Quart. Journ. Geol. Soc. vol. xxix. 1872. 
* First described by Prof. Allman (Quart. Journ. Microscopical Science, n. s. vol. ix. p. 57), 
next Ossian Sars (id. x. 3 . vol. xiv. p. 1), previously Ray Lankester has published some valuable 
“ Kemarks on the Affinities of Rhabdopleura ” in’the same work, id p. 77. 
6 Joh. Müller, Ueber die Larven und die Metamorphose des Echinoderm, p. 22, Taf. 2, fig. 1, 
1849. 
