﻿266 T. Davidson — What is a Braqhiopod ? 



affinities of the Bracbiopoda have been kindly communicated to me 

 by Prof. W. King, Sc.D. :— 



" The group PalUobranchiata embraces forms formed of two tj^es 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 

 thoughts should be directed to the simplest of these divisions : but a difficulty 

 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, Luidia, etc.). Adopting this supposition, the Clistenterates may be 

 dismissed from fiu'ther consideration. I shall, therefore, regard the Tretenterates 

 as the initial or older type. The Cambrian system is not only the first in which 

 indisputable organic remains occur, 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 ' [Ortk/s Sicksii), 

 being all more or less related to Limjula and Discina. 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 

 afiinities of any group of organisms can only be determined by its structural charac- 

 ters being considered in connexion with those of other groups of contemporaneous 

 (geological) origin,^ 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 memoir on the Tremadoc rocks in the neighbour- 

 hood of St. Davids, in South Wales ; ^ — 



Frotospongia Spongida. 



Oldhmnia \ " o -n i 



I i rolyzoa. 



Theca ? Pteropoda. 



Liiigulella, etc Palliobranchiata. 



Falasterina Asteridia and Ceptoidia. 



Histioderma, Scolithus Annelida. 



Paradoxides, Agnostus, etc Crustacea. 



Of these groups the Palliobranchs have often been associated with Folyzoa ; but 

 lately Morse has endeavoured to show that they are more closely related to one of 

 the others, Aimelida. There is no doubt he has succeeded in adducing certain points 

 in his 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 

 Rhdhdoplcura, a marine form of Hippocrepian polyzoa.* There is another group of 

 organisms, also occurring in Cambrian rocks, which does not appear to have attracted 

 much notice in connexion with the subject in hand — I allude to the Aateridia. The 

 late Johannes Miiller showed that in the larval state certain star-fishes have their 

 form completely difi'erent 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 asteriget'a), which possesses features strongly reminding one of the structure of 

 Liiigula. 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.*^ 

 This seems so very like the general sti'uctural plan of Lingula that I prefer asso- 



1 An Orthis (possibly a Clistenterate) has been found in the Menevian g'roup, but to whatever 

 division it may belong does not matter, as LinguleUn occurs at the base of the Cambrians. 



2 See a memoir by Prof. King-, entitled, ''An Attempt to Classify the Tetrabranchiate 

 Cci)liiilopod.s." Annals of Natural History, in 1845. 



» Quart. Journ. Geol. Soc. vol. xxix. 1872. 



* Fir.st described by Prof. Allman (Quart. Journ. Microscopical Science, n. s. vol. ix. p. 57), 

 next Ossian Sars (id. n. -3. vol. xiv. p. 1), previously Ray Lankester has published some valuable 

 " Remarks on the Affinities of Rhabdopleura " in the same work, id p. 77. 



* .Joh. Miiller, Ueber die Larven und die Metamorphose des Echinoderm, p. 22, Taf. 2, fig. 1, 

 1849. 



