The variation in the number and direction of the spines is a striking feature in Spondylus. When the whole lower surface adheres to branches of coral, a very frequent occurrence, they are confined to the upper valve, but when a part only of the valve, the whole surface becomes covered.
Having finished our short sketch of the Conchifera, we shall now treat of the singular group, Brachiopoda,[11] which some place nearer to the Gasteropoda than the Pteropoda, giving them, in fact, their place. It is out of the province of this work to enter into the physiological arguments of such a question. The days of the Brachiopoda or short-footed animals are past. Of the 1842[12] species formerly known, a few types of a small number of genera only are left, numbering in all 102. The Terebratulidæ are best represented; there were once 300 or 400 species; there are now not more than 67 in the seas of the world. The difference between the past and the present is especially striking, when we compare the recent and fossil species of Europe. Among no other class of shells has there been such a wholesale extinction of species. The great family of Spiriferæ are wholly extinct, and of 400 Rhynconella only four are now living. The curious Crania, Discina, and Lingula are still living, and are mostly found in the seas of the southern hemisphere.
CEPHALOUS MOLLUSCA.
CHAPTER XIII.
GASTEROPODA.
We shall now consider the Gasteropoda, which is divided into four orders. Firstly, Nucleobranchiata, animals which float on the surface of the ocean: they are Diæcious, or in separate sexes, and the nervous system is widely distributed in the body, the shell, in Carinaria, for instance, covering only a very small portion of the body. The first family of this order is Atlantidæ, of which the types are the fossil Bellerophon and the recent Atlanta.
The second family is Firolidæ, the types of which are Carinaria and Firola. Carinaria or glass nautilus is shaped like the bonnet-cap shell, Pileopsis. It is as transparent as glass; and although now very common, was formerly one of the most highly-prized shells by collectors. The second order of Gasteropoda is Opistho-Branchiata, and is divided into two sections, the Nudibranchiata, and the Tectibranchiata. The Nudibranchiata have no shell except in the larva state; they mostly live at the bottom of the sea on rocky shores, but a small number swim on the surface. They are remarkable for their variety of form and vivid colouring, being the most beautiful of all molluscous animals; they may truly be called the caterpillars of the sea, for their branchiæ remind us of the spines with which many lepidopterous larvæ are covered.