128 LECTURES ON MOLLUSCA. 



Family CRANIAD^S. 



The Oranias have lived from the palaeozoic times till now. They 

 have no hinge, and are attached by the front valve : the back valve 

 being limpet shaped. The mouth-arms are free, supported by a nose- 

 like projection in the front valve. The eye-like muscular scars give 

 some of the species a rude resemblance to a skull. The valves are 

 shelly, and very minutely punctured. The ancient Pseudocrania had 

 the valves free. The position of Spondilobolus is uncertain. 



Family DISCINHXZE. 



The shells of Discina are quite horny, and flexible when fresh. 

 They are attached by a peduncle, passing through a chink in the 

 lower valve. The mantle is surrounded by stiff bristles ; but the cilia 

 on the mouth-arms are very tender and flexible. The ancient fossils 

 have been separated as Orbiculoidea. TrematishsiS convex valves, with 

 a thickened hinge-margin. Siplionotreta is covered with hollow spines, 

 with a tubular hole at the beak. Acrotreta is shaped like Calceola. 



Family LINGULID.E. 



As the Lingulas are the earliest,, so they may be regarded as the 

 lowest bivalve shells. They live half buried in sand or mud, often at 

 slight depths ; and, as their horny shells hang at the end of a very 

 long peduncle, they have no slight resemblance to the Lepad Barna- 

 cles. Members of the group lived in all ages in the British seas, down 

 to the Coralline Crag ; and a species is still living on the Atlantic 

 shores of North America. The Silurian form Obolus is nearly round, 

 with a thickened hinge-margin. 



CLASS TUNICATA. 

 (Tunicaries, or Cloaked Mollusks.) 



We have now completed our sketch of the shell-bearing classes of 

 Mollusks. The remaining groups form a transition to the zoophytic 

 condition of animal life. The higher Tunicaries offer many points of 

 similarity with the sedentary Lamellibranchs ; but the lower races 

 lose their separate individuality, and become incorporated into a gen- 

 eral mass of life, like the Polypes. Although not attractive to the 

 general observer, they present many points of singular interest to the 

 scientific student. They have lately been carefully examined and re- 

 ported on by Huxley and Rupert Jones. The first group are the soli- 

 tary or simple Ascidians. 



Family ASCIDIADJE. (Sea-Squirts.) 



The Sea-squirts appear at first sight nothing but leathery bags, 

 covered perhaps with sea-weed or other accretions. The presence of 

 organic life is only made known to us by the violent jets of water 

 which they force out when disturbed. This leathery bag or " test" 

 takes the place of the shell in the bivalves. It is less distinctly ani- 

 mal in its nature than any other substance produced by sentient life, 

 containing a large quantity of the vegetative cellulose. It is freely 

 bored into by .bivalve mollusks, such as Crenella and Mytilimeria. But 

 under this test, is found a delicate mantle, like that of ordinary mol- 

 lusks, united into a sac, and terminating in two openings, the inha- 



