GASTROPODS. 



365 



about high-water mark. The shells are distinguishable by a groove on the right 

 side of the inner surface, interrupting the muscle by which the animal is attached. 

 Siphonaria much resembles Auricula as regards the pulmonary cavity, which is 

 adapted for breathing air or water, there being both a lung and a modified o-ill. It 

 is, however, protected or closed by a valve-like lobe of the mantle. The horny 

 jaw has a smooth and curved cutting edge. The radula consists of a single central 

 or rachidian tooth and numerous laterals, very similar in type to that of certain 

 groups of Pulmonata. About fifty species are known. The genus has an extensive 

 range from Vancouver Island in the north to Cape Horn in the south. One species 

 occurs on the coast of Portugal, but the majority inhabit eastern seas. In Gadinia 

 the breathing-chamber is without any sign of branchiae. It is a limpet-like animal 

 with a circular foot by which it attaches itself to rocks or stones, in the same 

 manner as limpets. It is probably a poor traveller. The pulmonary orifice is 

 placed on the right side near the head, and is closed by a small valve. The head 

 is large, without tentacles, with the sessile eyes placed at the sides. Mr. Dall, who 

 watched a colony of G. reticulata, noticed that " as long as the rock on which they 

 were remained damp, they continued with the margin of the shell firmly applied 

 to it. As soon as the boulder became dry, under the hot sun, I perceived a simul- 

 taneous motion in the colony. Each shell was raised above the surface of the 

 stone, the head and foot were protruded, and the orifice of the pulmonary cavity 

 was expanded. They were evidently enjoying the warm air." 



Suborder Pteropoda. 



Until within the last few years, the molluscs included in this suborder were 

 considered to constitute a separate class. Some systematists located the group 

 near the Cephalopoda, others, believing them to be of a more degraded type than 

 the Gastropoda, assigned them a position between that class and the bivalved 

 Mollusca. Dr. Morch and others, long ago pointed out the affinity of the 

 Pteropods with the Opisthobranchs, and Dr. Pelseneer has come to the con- 

 clusion that these molluscs should be included in two of the Tectibranchiate 

 groups of the Opisthobranchs. The Pteropods seldom come near land, except 

 when driven by currents and tempests, and although they rise to the surface 

 principally at night, they have been observed during the daytime. They are 

 characterised by two delicate fins or lateral expansions of the foot (parapodia), 

 placed at the anterior end of the animal, and used in swimming, being moved with 

 considerable rapidity, like the wings of a butterfly. When they wish to sink, 

 their fins are contracted, and the anterior part of the body, in some cases, is more 

 or less withdrawn into the shell or the mantle, and they thus fall to the depth 

 desired. At times they remain stationary, by keeping the fins merely extended. 

 Some — Clio, Pneumoderma, for example — adhere to floating bodies by means of 

 oral appendages provided with suckers, others by means of a sufficiently developed 

 foot. The fins, termed epipodia by Huxley, are sometimes united, forming a single 

 disc, sometimes partly connected, or, finally, entirely separated. The Pteropods 

 are hermaphrodite, or have the sexes united in each individual. They deposit their 

 ova in the form of long bands which float on the surface of the sea. They are 



