362 Mr. Robert Garner's Malacological Notes. 



ship to the environment, and on the other unity of plan 

 throughout. What is a little remarkable in this last example, 

 instancing- a kind of duplex structure, is that in the develop- 

 ment of thecirripede the articulated disposition is primary and 

 internal, the molluscan (if such it maybe termed) the reverse. 

 Darwin considers that the cirripedes present no real affinity with 

 any other non-crustaceous animals ; but some have seen in their 

 pedunculated and sessile genera an affinity, on the one side, 

 to the Brachiopoda and the Rudista respectively, and in these 

 last to certain ordinary mollusks on the other. The first con- 

 clusion we scarcely admit ; the last we do. 



The ciliated tentacular crown or lojjhophore of the Bryozoa 

 much more resembles that of some tubicolous annelids than 

 the branchial cavity of the Ascidia, though the young bryo- 

 zoon {Plumatella) is somewhat like the bivalve in the early 

 stage, the egg-covering sepiirating into two oval plates or 

 valves. There are other external resemblances of MoUusca 

 to lower or higher animals, analogous rather than homologous 

 or homogenetic, but brought about purely to meet peculiarities 

 of the circumstances of life (that is, special excess of those 

 externals which nevertheless are set down as determining, in 

 a gradual way, all animal development whatever) ^ either by an 

 adaptation to, or a modification by the same. Such are the 

 resemblances of some aberrant mollusks to tubicolous annelids, 

 or of a Chiton to an Oniscus — resemblances totally disregarded, 

 but whether philosophically so some might doubt. 



The mollusk, notwithstanding its frequent tendency to a 

 flexure supey- se, is at other times, when unfurnished with a 

 shell, of the more normal animal form, oval or elongate, with 

 an undivided foot or muscular disk below abdominally, a 

 protective mantle above dorsally, and the viscera enclosed 

 between. The form, however, is generally much modified to 

 accord with the shelly valves of the conchifer or bivalve, or 

 the spiral shell of the univalve. It is only as we ascend that 

 a head is developed (Cephalophores). 



The Ascidige, then, we are disposed to consider as the low- 

 est of the MoUusca and the progenitor of them, though not 

 of the Brachiopoda ; these are Acephala closely allied to the 

 others, the bivalves. The tentacles, at the orifice of the respi- 

 ratory sac, are, when they exist, but secondary to the sac, 

 which has the true branchial structure of the higher Acephala, 

 frequently with branchial or labial folds, like the pallial sac 

 of the bivalve. This difi'ers from the views of several living 

 biologists, but has the support of Mr. A. Hancock*. The 



* Journ. of I.inn. Soc. vol. ix. f 1868) p. 34.3. 



